Hello fellow summoners. I have wanted to write a guide for you guys, but I couldn't choose what champion I should talk about and, I admit, I was kinda lazy enough to delay it for a long while. However, without further ado, I present to you my Diana guide. I am not the most trustworthy player, as I don't play in a really high level of competition, but I have gained some mid-lane experience through my League of Legends progress and most of the stuff I'll write here are true. If you have any suggestions/questions or advices to make this guide and my future work better, please leave them as a comment below.
Also, if you feel I was not clear enough on some of my points, let me know and I'll fix that.

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Take into account that the Runes I tell you to use are open to being changed according to YOUR own preferences and play-style.
As I see this champion, the Runes I consider optimal for Mid Lane are the ones listed above, which are:

As it is possible that these runes are not the best, you can also consider using, for example, some Attack Speed Marks instead of the Magic Penetration ones, or considering Armor for Seals if you go against those AD bruisers. If you have any more suggestions regarding the Runes, contact me and I will look into it, and I might add them to the guide to spread your opinions.

I don't really have a structured idea for Top Lane Diana's runes, but I guess it would be around Armor ot HP Seals, Ability Power Quints, Magic Penetration or Attack Speed Marks, and Scaling Ability Power or Magic Resist Glyphs (varies depending on who you go against, and they don't differ that much from the ones I use for Mid Lane).

^th point in Veteran's Scars for the needed extra health.

For a Tanky Diana:
As I said for runes, I am open to discussion about the runes that should be used on a tanky Diana. The options would be 21-9-0 or 9-21-0, focusing on health and resistances and some AP. For now, I use

Flash: Pretty much a standard spell nowadays, can net you kills or let you escape from certain deaths.
Ignite: Another standard spell, has a good healing reduction effect and helps to secure that kill that you otherwise would let escape for so little.
Teleport: It is a viable spell for almost any Mid lane or Top Lane champion, overall pretty good for setting up ganks and kills, or just joining a teamfight from the other end of the map.

These three would be the ones I would usually go to, as they are the ones I use the most at this role. Another options to consider are:
Cleanse: Can be the deciding factor between life and death against a heavy CC jungler that you are afraid of or to just leave that deciding Amumu ult or anything of sorts.
Barrier: Offers a good shield and has a fairly low cooldown for a Summoner Spell. Overall a viable option.
Ghost: I don't personally like it on Diana, but I understand that it has a low cooldown and can be handy to chase an opponent down through great distances, increasing Diana's mobility.
Exhaust: Very situational, I would only consider this spell if I go against a Karthus to weaken his Defile or to survive a Crowstorm from Fiddlesticks (also applies to similar spells).

I don't really give it a thought to other spells, as I don't think they are that useful.

Let's start with the starting items. Since the start of Pre-Season 3, some people started going for "strange" starting builds, or so where them called. Turns out that those choices actually made sense and worked pretty well. My suggestions are:

• sight wardx1 x5
  Has been my main starting build since they nerfed pot start. Diana doesn't really need the mana regen, since her Q is very mana efficient, but it somewhat helps from level 6 and beyond.
  
• sight ward /
  This build pretty much serves the same purpose as the first one, but it is not that used since Crystalline Flask 's cost rose from 225 to 345.
  
• x5
  Go for this starting build if you go against a good AD harrasser, like Kha'Zix or Lee Sin.
  
• x3
  The standard starting build for season 2, allows you to dodge skillshots easier while still providing fairly good sustain.
  
• sight ward x4
  This build is mainly designed for a Top Lane Diana, as it provides good sustain, vision to prevent yourself from being ganked, an HP "steroid" if you get in trouble and makes farming easier with the extra AD after you pop the elixir.. You can always opt for an Elixir of Brilliance, but the Elixir of Fortitude feels much safer.
  
  
• Or
  I don't recommend these two paths, as they are kinda weak against those that rely on Regen and Sustain. The only times I would choose these items are if I know that I can win the lane easily and that I can escape jungle ganks, like a pre-6 Warwick.

There are various routes you may want to follow as Diana. Personally, I consider getting Sorcerer's Shoes, Zhonya's Hourglass and Abyssal Mask as my core items. I get them almost every game I go Mid, and if I play her Top lane, which is the minority of times, I consider buying a Haunting Guise first, if I realise that getting some penetration early will maximize my damage over getting Ability Power.

From here on out, you need to think what will your role be in the game. Wether it is to burst the enemy Carries, protecting your own Carries, or tanking enemy frontlines, you'll need to choose different Item Builds. I will only focus on the first two ones though, as those are the ones that I am more familiar with.

Burst Mage Build:
The build I often rely on for a bursty Diana is:
Sorcerer's Shoes, Abyssal Mask, Zhonya's Hourglass, Rabadon's Deathcap, Nashor's Tooth and Liandry's Torment.
Regardless, I can't stress enough that this is not a definitive build, it is up to YOU to decide wether you change it or not, and adapt to the team you are playing against.
As Situational Items for this build, I would recommend that you consider getting a Lich Bane for an extremely fed AD Carry that is an enourmous threat to your whole team, as you would be able to burst him/her down much quicker; a Guardian Angel if you decide that your team needs that "extra player" for teamfights; or a Deathfire Grasp for the same reasons that you would want a Lich Bane plus the CDR.

The Tanky "Bruiser" Build:
I recommend going for this build generally for a Solo Top Diana or if you decide that your role will be protecting your carries and make the enemy frontlines busy during teamfights, even if you are playing Mid. I prefer going for the following items:
Sorcerer's Shoes OR Mercury's Treads, Abyssal Mask, Zhonya's Hourglass, Rod of Ages, Rylai's Crystal Scepter and Iceborn Gauntlet.
You may be asking yourselves why I get Iceborn Gauntlet here, and it is because this item is amazing for frontline tanks and has a very powerful slowing AoE that deals decent damage.
Another question may be "Why both RoA and Rylai's?". Well, I find having both an amazing benefit: Rod of Ages gives you a good amount of sustain if you decide to go for it early and its passive gives you fairly good stats, while the Rylai's Crystal Scepter gives you yet another slow and amazing HP for the role you are fullfilling.
You can also opt for a Randuin's Omen for its great stats and the amazing active, Liandry's Torment for that good penetration, or even opt to trade RoA and Rylai's for the Sunfire Aegis + Warmog's Armor if you think you don't need that much damage and prefer the tankiness this path gives you. Again, it is up to you to decide which build adapts better to the game you are playing and to your play-style.

As you probably saw in the beggining of this guide, I opt to max Crescent Strike after getting 2 points in Pale Cascade and 1 point in Moonfall at level 5. The priority order for the standard build would be: R>Q>W>E. The reason why I get 2 points in Pale Cascade early is because I feel a bit safer having them, as it provides a good shield that can be refreshed once and may be decisive in early trades in your lane.
You may argue that you should max Pale Cascade first in some situations, and I would if I thought it would be approppriate (e.g. being Top Lane against a Cho'Gath or a Yorick, whom harrass you non-stop and have both good sustain), but maxing Moonfall first has no benefits at all, since having it more often without the damage would be close to useless.
You can also try to max Crescent Strike and Pale Cascade evenly (while leaving Moonfall at one point and getting Lunar Rush whenever you can), it's and interesting build that may or may not work, you can go ahead and test it as I didn't really tried it enough to have my own point.

Diana is an unique champion in the league. She has the only skillshot that travels in a curve and not in a straight line, a shield that refreshes itself if all the orbs explode, and her ult, that is not that unique as it is similar to Akali's Shadow Dance.
You should use these unique characteristics to your advantage whenever possible, and try to understand the champion's strenghts and your opponent's strenghts to judge how many damage you can do at a given time.

Pros: High Mobility; Very High Burst; Has good survivability with Pale Cascade; Somewhat Versatile; Crescent Strike is very mana efficient and offers good farming/pushing/harrassing potential.

Cons: Can be stopped if denied early farm; Spells that stop initiations can screw her up. If you opponent can dodge your Q most of the times, you'll lose some of your potential

Farming as Diana is one of the stuff that I consider extremely important to be successful at. If you farm well enough, you'll be able to get your items earlier, improve your potential to roam and gank other lanes, thus giving your team some kills. Bare in mind that you have your Crescent Strike, that can help you get a minion kill that is not safe to walk to and auto-attack to get.

For early levels, you should try to farm as much as you possibly can, and around level 6 ask your jungler to let you farm his wraiths. With a Q+R combo you can get there no time and you can clear the camp quite easily with your skillset. Having access to blue buff is also very important, as it will allow you to spam and spam your Q even more, lending you easy farm.
What you will want to do near the end of the laning phase is push your lane, gank one of the side lanes and, if you are successful, try to take global objectives such as Turrets or Dragon.

When mid-game starts, you will, generally, slowly decrease your farm rate. Try to farm a lane when it gets pushed and group with your team again as soon as possible. You can also farm the jungle to help you hold your advantage or to get closer to the enemy team.

Late-game you should focus on staying with your team and doing all you can in teamfights, that will be either bursting one of the carries, and here you should realise which one of them has more influence in teamfights; or keeping the enemy fronline busy enough so that they can barely touch your carries.
Bursting enemy carries:
Try to land a Crescent Strike on your target. Although your goal is you burst ONE enemy, you should try hitting as many enemies as possible with the skill. After you successfully hit one, your team should realise that it is time to engage. Jump on your target with Lunar Rush and pop Pale Cascade as soon as you get to him/her. Try to get three AAs off and finish the job with another Q, while saving your Moonfall to when they try to run away from you and your second Lunar Rush to jump to another target that you prioritize after you get your Q up again or to simply back off to an enemy if you realise you can't burst them that quicly, either due to over-protection or because you're still not strong enough.

Keeping the frontlines busy and protecting your carries:
If you realise your role is to protect that Kog'Maw or that Vayne on your team, whom can both deal a lot of damage if given the freedom to, you should prioritize the enemy frontlines or those champions that can jump right into your carries and burst them down before they can react, like Kha'Zix, Akali, Katarina, etc, you get the idea. Use your Moonfall to pull them to you and slow them when they get too close, and QR combo to get to them when Moonfall is on cooldown. Your Rylai's Crystal Scepter and Iceborn Gauntlet come into play here, as the slows will be important to keep your carries free and alive. After all the threats to the carry are gone, there is only one thing left: clean up. As an assassin, Diana is pretty good at cleaning up the battlefield after danger is gone, wether it is a CC ability or a scary champion. Just go and kill what's left if you have the HP or Mana to do it. Actually, even if you don't, you still should, as it will provide safety to the target you want to protect yet again.

In this section I'll try to describe how should you act against different lane matchups. I do not have intentions to cover every single champion you can go against, I'll try to give you some basic knowledge on how you should act against different lane situations.

Firstly, we have skillshot champions.
Against these, you should put some effort into dodging Orianna's Command: Attack or Cho'Gath's Rupture whenever they use it to harrass you. Bare in mind that skillshot champions will use their skills if they see an opportunity to harrass and farm at the same time, which can help you baiting these cooldowns if you want to try something. Another important note is that most of these champions will have a really hard time once you jump on them, so try to capitalize on this fact to score a kill or just harrassing them out of lane.

Now, single-target damage champions.
Against STD champions (not the best acronym/abbreviation here, but it was intended), such as Ryze, Kassadin and Akali, you should focus on trying to harrass them before they harrass you. What happens most of the times is that they'll try to poke you down to set you up for an easy gank or getting a kill themselves. You should know when you can deal more damage than them and if you have enough sustain/survivability to survive a lane like this in early levels. When you reach level 6, there are few champions that can handle your damage, but some of these single-target damage focused champions may be able to kill you once you throw your combo. Akali can hide in her Twilight Shroud and wait till your Moonlight debuff wears off and kill you after that, Kassadin can just Riftwalk away. DO NOT over-estimate your damage against STD champions (HUE) as it can reveal a bad mistake.

In this section, I would also like to cover how you should act against champions with different ranges.
For Melee champions:
These are the ones that have the same AA range as you (Duh!). To gain some advantage in the lane, you should try to harrass with Q when they get into melee-range to get a minion kill. When you feel they are low enough, do your thing and burst them down. Be careful, as this advice also applies to the other side, your opponent, meaning that he/she will try to do this as well, so use what you know about this method to avoid it.

For ranged champions:
Most ranged champions are pretty squishy, but they can be a pain in the a** sometimes. Champions like Lux can sit at a safe range while being able to farm evenly. If there are any trades between you, take them if you know that you can deal the damage while either dodging their damage or simply absorving it with Pale Cascade. You should be able to win past-6, since they tend to be squishy enough to burst them with Q-R-W-E-someAAs and finishing them up with another Q or the second R.

Last but not least, what can you do if you get "countered"?
Well, getting countered is not the end of the world. If you feel you are getting pressured really hard you should stay safe, farm as much as you can against these advert conditions, and be sure you are ready to teamfight. Getting countered in lane does not mean you lose the lane, it depends on how well you know the match-up and both champions. Some people say LeBlanc counters Diana, or that you'll lose if you face a Mordekaiser, but there is a chance that you can win, and I already did. If you think you have no chance on your own, ask for your jungler to pressure the lane, denying your enemy.

As many of you know and some complain about, Diana is a really strong champion. She will probably be nerfed soon enough, but that does not mean that you should stop playing her. In my opinion, it's not her scaling that makes her strong, it's her unique style. You may argue that every champion has its own "Unique" style, but it may not be as evident as it is with Diana.
Note that the best way to learn a champion is not through reading guides, although they help. So stop reading guides (EXCEPT MINE) and go try your own builds and strategies! Experience is the best way to improve!

I hope you liked this guide. If you notice any mistakes or errors, let me know, and contact me if you have any suggestions or opinions that you would like to share to help me improve.
Thanks for reading and I hope you have a great League of Legends journey.

Best of luck in the Fields of Justice!
Markmata.

23:40 CET (Central European Time) - March 23^rd, 2013: Published guide.
10:58 CET (Central European Time) - March 24^th, 2013: Changed masteries and Runes for Build 1, Runes for Build 2, corrected some mistakes. Mastery explanation coming soon along with some pictures and more formatting. C2V added. Chapter order updated.
20:55 CET (Central European Time) - March 24^th, 2013: Masteries added! For some reason, the part that said ""OP" Champion" in the guide's title kept getting deleted, so I changed it.
01:24 CET (Central European Time) - April 13^th, 2013: Fixed minor errors.
18:03 CET (Central European Time) - June 2^nd, 2013: Updated Items section.