I have played trundle for a bit now, and i have liked so many things about him. This is my guide on how to play Trundle, both offensive and defensive. This guide is going to tell you about DPS and Tanky DPS trundle styles. I do either of them depending on enemy teams. Tweaks and edits on the build will be made if a better suggestion has been made.

Please leave notes and suggestions for me. Please don't give me a Thumbs Down if it doesn't work out but give suggestions. Unless it's epic failure in your eyes, then you have my blessing to give me those -1's.

Pros
-Jungler
-High Crowd Control Reduction
-Tank shredder
-High survivability
-Super hot, and trolls for a living

Cons
-Can't tank whole team
-Melee
-Doesn't have a stun
-Requires common sense

Do's
-Initiate WITH your tank
-Use Agony their Armor/Magic Resistant stacking tank and bash down their carry
-Jungle until level 6 or have a chance for easy kill
-Gank whenever theres an open spot to
-Look sexy and pose for jungle monsters

Don't
-Initiate if you're not going really tanky Dps style
-Use Agony on squishy champions. (unless they have under 1 bar and is out running you)
-Stop trolling
-Thumbs down this guide

TIME FOR A PICTURE. ;)

My runes are 4Greater Marks of Might, 5Greater Marks of Desolation, 9Greater Seals of Resilience, and 9Greater Glyphs of Focus.

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4 Greater Marks of Might. When jungling, you need the extra damage so you wont take extra damage.
If not jungling switch these for 4 more Greater Marks of Desolation
5 Greater Marks of Desolation. During Mid - Late game you can not depend on pure damage to DEAL damage. When opponents have all these fancy items to block off your damage, you need as much armor penetration as possible that you will lack in items, unless bought.
9 Greater Seals of Resilience. As a melee Tanky/OffTank Dps you will need all the help to survive in a team fight as possible. You'll be rushing in, throwing your ult on the highest armor champion on their team and bashing down the carries, in the process you will be kicked down to size because of your melee disadvantage. So you need the armor.
Greater Glyphs of Focus. Such an amazing ultimate, so much CD. With 5.85% Cool Down reduction you'll be spamming your ult every 50 seconds.
On the side, you can replace these runes with any blue runes you desire because blue runes do not focus on helping you dish out damage, or beef you up to tank.

Jungling
1/19/10.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~Offensive~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In the offensive tree I got Plentiful Bounty. It helps with farming and shortens the cooldown by 5 seconds.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~Defensive~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In the defensive tree i got Resistancechained into -> Strength of Spirit. I personally like having the extra 2hp regen per 5 second at the start and having more by the time the items are finished. I grabbed Hardiness-> Harden Skin, for defense against jungle monsters, it is a BIG help. Having Defensive Masteryis self-explanitory, you're a jungler. Lastly i have Evasion-> Nimbleness. They work wonderfully together as one give dodge chance and the other gives movement speed after a successful dodge.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~Utility~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Since you have ghost Hastehelps a bunch. Take Perseverancefor the extra regen. Awarenessis a MUST for junglers, if you don't have this after killing golems you will remain at level 1. Utility Masteryis self-explanatory, you will be getting buffs and the 30% extra duration is like god for Trundle.

Laning
9/21/0
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~Offensive~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Cripple, you have Exhaust, this is self-explanatory. Deadliness, extra 2% crit = extra 2% chance of faster killing... Alacritygives nice attack speed boosts, good with finished items. I can't resist having Sunder's2/4/6 armor penetration.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~Defensive~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
For laning mastery's defensive tree i get Resistance-> Strength of Spirit. Max MP -> Hp Regen per/5sec late game is good. I took Hardiness-> Harden Skin, for the minor last hit aggravation i might lure so taking little damage is good. I have Evasion-> Nimbleness. Once again they work good together as one give dodge chance and the other gives movement speed after a successful dodge. For a bit of extra hp boosts i maxed out Veteran's Scars, and to finish i got the 4% total damage reduction, Tenacity.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~Utility~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Blank... (:

Jungling/Dps
Vampiric Scepter- The Life steal is mandatory if you want to jungle and stay alive until high levels. -> Wriggle's Lantern.
Wriggle's Lantern- 20% to deal 500damage to minion / monster. (life steal does work with effect) Perfect for a jungler. Also a free ward every 3 minutes, wards last 3 minutes.
Mercury Treads- Stacks with your Contamination causing you to basically have no CC effects on yourself while you are on the contaminated area.
Phage- 1 of 4 hits can cause an effect of a 30% slow on your target. Gives health, damage, and slow. Perfect. -> Trinity Force.
Sheen- Works and stacks with Rabid Bite, dealing Base Damage x 2 + 30/45/60/75/90. Gives mana, ability power..., and extra damage. -> Trinity Force.
Zeal- Gives Movespeed, Critical Strike chance, and provides attack speed. -> Trinity Force.
Trinity Force- 1 of 4 hits cause an effect of a 35% slow on your target, after a spell cast your next attack will deal 120% extra damage. Gives health, Crit chance, mana, movespeed, attack speed and damage (+ Ability Power).
Sunfire Cape- Deals 40 damage per second to units around you, gives great armor, and health.
ALTERNATLY for Sunfire...
Banshee's Veil- This is better suited if they have a heavier magic damage team over physical damage.
Phantom Dancer- 30% crit chance, 55% attack speed, and 15% movespeed.
Infinity Edge- 20% crit chance, 75 Damage, and gives 200% -> 250% crit DAMAGE. Perfect with Phantom dancer's crit chances.

Laning/Tanky DPS
Doran's Shield- Essential for staying in lane as Trundle, he is just to aimed at due to his lack of armor, range, and lane harassing ability.
Phage- Helps team slow enemies, mostly used for support. Gives HEALTH, damage, and slow. Perfect. -> Frozen Mallet.
Mercury Treads- Perfect for Tank Trundle because of the extra Crowd Control reduction effects that stack with the CC reduction effects of your Contaminate.
Frozen Mallet- Strengthens your ability to tank as well as for sure slow to ensure a good chase. Supports really well for your team, especially when/if you initiate a team fight. I love this item because it stacks with your Pillar of Filth.
Sunfire Cape- Gives armor and health for tanking, obviously, but also helps to dish out aoe damage since you will probably be melee smacking dps/tanks down slowly.
Banshee's Veil- Banshee works great when enemies have many Crowd controls that can be chained together for a unlimited stun. This will save your life from global damage spells when it is up and shielding.
Guardian Angel- You won't be indestructible but with Guardian Angel you will be 10 steps closer to it. Reviving is a crucial part in not dying, especially when you have healing spells (Agony).
Blood Thirster- Lastly i picked Blood Thirster. When you don't die easily, you won't lose stacks on this baby easily. Instead of just tanking towers or teams, you can help by dealing a lot more damage.

SITUATIONAL
You can replace many items in this build. Like for...
Offence
Last Whisper- This will work great when enemies stack armor for you.
Black Cleaver- Gives nice attack speed and helps well with the armor shredding for your team's advantage against the enemy team.

Defence
Force Of Nature- Gives additional move speed and regeneration but what's best is the magic resistance that it helps give.
Randuin's Omen- Helps quick escapes or ranged kiting champions. Slows down people attacking you and it's active slows everyone around you. Over Powered against ranged carries.

SMITE
Obvious reasons. Getting the extra damage is the BEST SH** IN THE WORLD, for a jungler.
GHOST
You have good natural move speed already but no leap/jump skills to catch up to someone. Your Contaminate has a limited range so you don't get too much catching up to most characters. Ghost is your solution.
Exhaust
Alternately Exhaust will work fine. Especially against damage stackers, you will shred them with exhaust.

For Jungling i follow Stonewall008's Trundle jungling route. (my favorite. might require runes/masteries.) Play Video
Alternatively this offensive allowing you for better early game ganks. BEST FOR BEGINNERS! Play Video
If you like to jump from camp to camp without hesitation... Play Video

Rabid Bite
Always use Rabid Bite on their DamagePerSecond (DPS) character. This is one of the best things to do for your team. You 'sap' their damage, increase yours and deal extra damage. When you have your semi-tank items to keep you alive, rush away and aim them!
Contaminate
Stay in this ring of dirty floor because you gain so many great effects in it. Attack Speed, Move Speed, and Crowd Control Reduction. Stay in this during team fights. Always fight near a corner of it because when need to escape, the extra Move Speed will allow you to just burst away going the opposite side.
Pillar Of Filth
Use this to your advantage, ALWAYS. Save a friend, block a pathway, team up with an Anivia or manipulate the enemy one (trundle too), slow an enemy set a gank, or even use it to get an assist for fun.
NOTE: THIS SPELL WILL CAUSE TOWER AGGRO WHEN ENEMY IS SLOWED OR TOUCHED WITH IT.
Agony
It makes me sad when Trundles use this on a carry when they have high health. IT IS USELESS. This ability Saps 25% of all Armor/MagicResistance, doubling that over 6 seconds and dealing minor damage. Use it on a Amor/MagicResistant when they dont have a spell blocker up, you can get up to 500+ of both Armor AND Magic Resistance if you like to tank.
NOTE: USE THIS ON A CARRY THAT OUT RUNS YOU AND HAS <100 HP IF YOU LIKE. OR ELSE IT IS USELESS.

Remember, don't be forever alone and push solo 24/7. HELP YOUR TEAM.
DENIED, TRUNDLE OWNS. ALWAYS.

I will add warding spots. SOON TO COME.