What to expect -

What not to expect -

There are three mastery trees in LoL -

Offensive Tree

Summoner's Wrath :It gives you bonuses to all the offensive skills, which coincidentally are the ones people tend to use the most apart from Flash. In any builds using offensive summoner spells such as Ghost, Exhaust, or Ignite, this spell is a must-take.

Brute Force : Basic 3 points of attack damage. Used to last hit efficiently, common pick.

Mental Force : Basic 4 points of ability power. This mastery is usually picked my many guides for the AP, but I dislike it. Four points of AP is useless and three points of AD is more useful to last hit with. There are some exceptions though.

Butcher : 4 AD against creeps, useful for junglers reliant on attack damage, or in the lane for last hitting.

Alacrity : Attack speed, not much, but necessary for ArPen.

Sorcery: CDR, not much, but necessary for MagicPen.

Demolitionist : Not picked often as the extra damage against towers are rarely used and pretty pointless. Plus, the extra damage won't speed up a tower's death due to its armor, and how little the damage bonus actually is.

Deadliness : More AD, useful to get the crit bonus. It is scaling, so best for late game.

Weapon Expertise : Crucial skill in many builds, gives more ArPen which is great for AD reliant champs.

Arcane Knowledge : Crucial skill in many builds, gives more MagicPen which is great for AP or skill caster reliant champs.

Havoc : Gives 1.5% bonus damage upon AD damage and any skills you do.

Lethality : Gives you more crit damage WHEN you crit. Useful for those with skills that do 100% crit/higher crit chance, such as Ashe's Focus, or Shaco's Deceive.

Vampirism : 3% Lifesteal, it is very little, but during the laning phase, it is 2-4 more HP per hit, and with a Vampiric Sceptor and Doran's Blades, it can stack up lineally.

Blast : More AP at level 18, it is a scaling mastery. Good mastery for any AP caster for obvious reasons.

Sunder : Flat ArPen, it is useful for any AD based champs due to the effectiveness of ArPen. Early game, if you have ArPen runes, this works with it beautifully by shredding through squishy champs' armor.

Archmage : Fantastic skill, works like a Rabadon's Deathcap. It amplifies your AP by adding on 5% of total AP to your AP.

Executioner : Deals more damage as the target is low. It CAN be the reason you got a kill, or if you didn't have the mastery and you are the AD carry, you missed out on it. 6% more damage is pretty useful overtime, and that might be the few more damage you needed to kill someone. Works with spells too.

Defense Tree

Summoner's Resolve : Commonly picked by junglers for Smite bonus, and for AD ranged carries for the Heal bonus, you should look at the benefits it gives, see if any of your skills match the benefiting skill, and get it as seen appropriate.

Resistance : More MR for the early game. Get it against AP enemies in the lane.

Hardiness : More Armor for the early game. Useful in the lane more then Resistance since early game is largely about auto attacks, and minions do less damage to you.

Tough Skin : One of the best masteries for top champs and junglers as it reduces the damage done to you by minions. Less damage from minions, more health to trade with and survive.

Durability : Another scaling skill, useful for more health. It is usually gotten to get Veteran's Scars more so than anything.

Vigor : Tad more health regen, good for a bit longer sustain and to make up for minion damage.

Indomitable : One of the best masteries for all champs. Gives damage reduction (2) from all damage sources. Throughout the game, this will really pile up from all the minions, jungle creeps, and eventual lane harass. Great early and mid game for farming.

Veteran's Scars : A Free Health quint essentially. Good for any squishy champs, during the first few levels (1-4), this may be what saves you from a Ignite or from that one last hit from the enemy champ.

Evasion : Not commonly seen, helps you take less damage from AoE.

Bladed Armor : Best pick for lots of junglers, deals 6 damage back which can speed up your jungle by many seconds, especially on camps like Wraiths. Just make sure that you don't steal the Blue or Red while waiting to give it to someone. If you wait too long, you may end up killing it due to this mastery.

Siege Commander : Gives armor debuff for towers in a short range, but usually never usefully applied.

Initiator : Great for laning phase and later on before teamfights. Extra movement speed when you are high health. Laning phase - use the bushes and the mastery to avoid damage while wistfully juking and poking.

Enlightenment : Scaling mastery, gives decent CDR at level 18, and for some, it may be the only source of CDR. 8% is nothing to scoff at, and I enjoy getting it for many champs.

Honor Guard : Lowers 1.5% damage from all sources.

Mercenary : It is bonus flat EXP from kills/assists.

Juggernaut : Great final tier mastery, the 3% HP bonus is great for tanks and essentially is just free HP. The 10% Tenacity is fantastic, and with many champs like Irelia or Dr. Mundo, this skill helps them get rid of CC even faster.

Utility Tree

Summoner's Insight : Contains CD reduction for Flash, and many other spells that usually supports take. Again, check if your skills match the benefiting ones listed, and raise the mastery if seen appropriate.

Good Hands : Good skill, best late game when you can shave off 6+ seconds on your death timer, which may be what saves that Nexus Turret or get to that Baron. Best for those who die often as well such as tanks or supports.

Expanded Mind : Scaling skill. More Mana. Useful for many, especially champs with low mana pools but high mana usage such as Sivir or Malphite.

Improved Recall : Faster recalls, great for those with Teleport too.

Swiftness : 2% movement speed bonus, extremely useful since speed is a harder stat to get.

Meditation : A bit of mana regeneration, not a lot, useful for laning phase spell casters and supports.

Scout : If you buy wards, this is useful so best for supports. But 5% is very small, and unnoticeable in 95% cases.

Greed : More GP10, great for anyone. It is extra gold generated, especially good for supports who need more gold.

Transmutation : More Spell Vamp, the counter part to Vampirism . Due to Spell Vamp's more complicated formula and how little you actually heal from it, 3% isn't much.

Runic Affinity : Popular for many, it extends include Blue and Red buffs, and is an extra 30 seconds, which is pretty useful, especially for mid-lane blue dependent users. NOTE: This doesn't affect Baron Buff.

Wealth : You start off with a bit more gold which helps for those who want the extra health pot, or for supports to get that Vision Ward or more pots.

Awareness : More experience, great for solo lanes to beat the enemy, or for supports who lack experience due to all the constant warding.

Sage : More experience, best in the early lane phase when kills can snowball. Other than that, it loses effectiveness.

Strength of Spirit : Great mastery, you get HP regen according to your mana. Great for mana-stacking champs and generally a fantastic regen mastery. Best on Blitzcrank and Ryze.

Intelligence : 6% CDR, a useful thing. Helps you get max CDR on supports and any utility users.

Mastermind : Reduces your Summoner Spells' CD by 15%. Great skill overall, helps all summoner spells.