Dodge now been removed from the game.
Dodge items do not stack additively. For example, Jax, with level 5 counterstrike, has 18% dodge chance. If he then picks up ninja tabi (+12% dodge) his total dodge chance is 28% (and not 30%).
It's easiest to simply calculate the highest %dodge first (as they stack multiplicatively, order is actually irrelevant): In our case, the 18% dodge chance from counterstrike is simply added (18% of 100% hit chance is 18%) so Jax's dodge chance is 18%, and his "be hit" chance is 82%. Next, the doge chance from ninja tabi is added. 12% of 82% "be hit" chance is 9.8%, which then gets added to the previous 18%, for a grand total of ~28% dodge chance.
If there were another dodge item (say, an identical version of Ninja Tabi that stacked with itself), the +12% dodge would be 12% of 72%, or 8.6% increase.
The reason dodge is handled this way is such that the mitigation of dodge as a stat scales linearly; were dodge strictly additive, the more dodge% you had, the more damage further dodge% would reduce.
Casting ignite on an already ignited target will reset the duration of the DoT, losing any remaining damage the original ignite would have done.
I'm just going to quote from the "mechanics FAQ" from the official LoL forum.
Q: How do movement modifiers work?
(Credit to Larias)
A: First, you have your base movement
of say 300. You add boots first,
always boots first, so if you have
Let's say you get boots 2. That's 370
movement. Then, you get 35% movement
speed buff from items or runes or
masteries or buffs: that 370 is
multiplied by 1.35, or 499.5 movement
speed. If this is the only thing
affecting your movement speed, the
diminishing returns formula kicks in.
Now, let's say you get slowed by 38%
(ashe arrows). What happens is the
ORIGINAL 499.5 is multiplied by .62,
and you get 309.69 movement speed.
Because 309 is not above or below
diminishing returns, you stay at that
Q: What are these diminishing movement
returns? (Credit to Guinsoo)
A: For each point of movement you
would have above 490, that amount is
reduced by 50%. So if your final
movement speed was 748, it would be
reduced to 490 + (748-490)*0.5 = 604.
For each point of movement you would
have above 415, it is reduced by 20%.
So if your speed after all mods was
445, you would end with 415 + 30 * 0.8
Q: Do slows stack?
A: Yes and no. Slows from items do not
stack among themselves (Phage, Frozen
Mallet, Rylai's Crystal Scepter).
However, all other slows stack.
Just like the diminishing returns for
bonus movement speed, being slowed has
diminishing returns starting at and
And one addendum: The strongest slow/speed boost is always applied first.
Lifesteal has no diminishing returns. If you hit an opponent for 20 damage, and manage 200% lifesteal, you will regain 40 health.
+Attackspeed from items is additive and references the base attack speed of the champion. Buying two phantom dancers, for example, gives you 110% additional attacks per second, but because they are additive, the effect doesn't increase due to any other items you may have.
Attack speed has a hard cap of 2.5 attacks per second.
Armor and Magic Resistance
The effectiveness of Armor and Magic Resistance do not suffer from diminishing returns.
Please see this answer for a fully detailed explanation of why.