A base roll of 8+ on 2d (doubles re-rolling and adding) is required to hit someone in combat.
Warriors and Paragons add +1 to all combat attacks.
The level of an appropriate weapon talent is also added to the attack roll.
Crouching: -1 to attack roll, +1 DEF vs ranged attacks, move at 1/2 normal speed
Kneeling/Sitting: -1 to attack, -1 DEF vs melee attacks, +1 DEF vs ranged attacks
Crawling: -2 DEF vs melee attacks, +2 DEF vs ranged attacks, maximum move per round is MOVE score in feet
Prone: -2 to melee attacks, can not attempt most ranged attacks (except for using crossbows, then +1 to hit), -2 DEF vs melee attacks, +2 DEF vs ranged attacks
1/4 of body covered: +1 DEF vs ranged attacks
1/2 of body covered: +2 DEF vs ranged attacks
3/4 of body covered: +3 DEF vs ranged attacks
9/10 of body covered: +4 DEF vs ranged attacks
Someone in front of target (that you don't want to hit): +2 DEF vs ranged attacks
Someone to side of target (that you don't want to hit): +1 DEF vs ranged attacks
Light obscurement (fog in melee, moderate darkness, light foliage): +1 DEF
Moderate obscurement (fog (ranged attacks), near-total darkness, dense foliage): +2 DEF
Heavy obscurement (pitch black darkness, defender is invisible): +4 DEF
A few inches high/long (small bird, frog, archer's bullseye): +3 DEF
About a foot high/long (small dog, cat, human head): +2 DEF
2-3 feet high/long (adult human limb, medium dog) +1 DEF
Twice human size: -1 DEF
Around 15-20 feet tall: -2 DEF
Holy crap, that sucker's ginormous: -3 DEF
(various combat maneuvers and other modifiers to follow)