Miss

A miss is a failed attack; it deals no damage and will not proc any mechanic that may have resulted from a successful attack.

Base Miss Rate
The Base Miss Rate for White Attacks is calculated using the difference between the attacker’s Attack Rating and the defender’s Defense. It is 5% if the Attack Rating is equal to the Defense Skill. If a player is Dual Wielding, they inherit an additional 19% miss chance. This applies to mobs, as well. Prince Malchezzar crushes so often in Phase 2 because the additional miss chance pushes non-crushing attacks much further off of his attack table.

The Base Miss Rate for Yellow Attacks is at a maximum 5%, not including any debuffs. The Dual Wield Penalty does not affect the miss rate for yellow attacks.

Player/Mob vs Player
When a player or mob attacks a player, the base miss rate is 5%.

For each point of the defender's defense skill over the attacker's attack rating, the base miss rate increases by 0.04%.

For each point of the attacker's attack rating over the defender's defense skill, the base miss rate decreases by 0.02%.

Player vs Mob
In PvE, a player's chance to miss is determined solely by the difference between the defending mob's Defense Skill and the player's Weapon Skill. There are actually two formulas to determine miss chance against a mob:

If the difference between the mob's Defense Skill and your Weapon Skill is less than or equal to 10 then the formula for calculating your base miss rate against that mob is:

5% + (Defense Skill - Weapon Skill)*.1%
 * If dual-wielding, substitute 24% for 5% in this formula

If the difference between the mob's Defense Skill and your Weapon Skill is greater than 10, then the formula for calculating your base miss rate against that mob is:

6% + (Defense Skill - Weapon Skill - 10)*.4%
 * If dual-wielding, substitute 25% for 6% in this formula

These data show that a level 80 character with maximum base Weapon Skill (400) will need a Hit Rating of 263 (8.00%) to theoretically never miss a Level 83 mob (or skull boss). It is possible to completely negate melee and ranged melee miss. (There used to be a hard floor of a 1% Miss chance on spells, but this was removed in 3.0).

Level is not a factor except when attacking a mob level 9 or lower. Miss chance against a low-level mob is a percentage of normal equal to the mob’s level divided by 10.

Calculating a mob's Defense Skill or Attack Rating

This is a rather simple formula. It is the Mob's level multiplied by 5.

For Skull Bosses, the formula is your level plus 3, multiplied by 5, with a minimum based on when the Boss was introduced (Bosses introduced in 1.x have a minimum level of 63, those introduced in 2.x have a minimum level of 73, and those introduced in 3.x have a minimum level of 83).

Other ways to miss
A ranged attack can display a result of Miss if the target leaves range, breaks line of sight or uses Vanish while it's in the air.