Touch Spells and Combat (PHB)

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Many spells used by priests and wizards take effect only when the target is touched by the caster. Under normal circumstances, this is no problem—the spellcaster reaches out and touches the recipient. However, if the target is unwilling, or the spell is used in the midst of a general melee, the situation is much different.
Unwilling Targets: The spellcaster must make a successful attack roll for the spell to have any effect. The wizard or priest calculates his to-hit number normally, according to the intended victim's Armor Class and other protections. The DM can modify the roll if the victim is unprepared for or unaware of the attack. If the roll succeeds, the spellcaster touches the target and the normal spell effect occurs.
Willing Targets: When attempting to cast a spell on a willing target, the casting is automatic as long as both characters are not engaged in combat. For example, if a fighter withdraws from melee, a cleric could heal him the next round.
If the recipient of the spell attempts to do anything besides waiting for the spell to take effect, an attack roll against AC 10 must be made. However, no AC modifiers for Dexterity are applied, since the target is not trying to avoid the spell!
Whenever a touch spell is successful, the spellcaster suffers from any special defenses of his target, if they are continually in operation. A successful touch to a vampire would not result in energy drain, since the power only works when the vampire wills it, but touching a fire elemental would result in serious burns.
When a touch spell is cast, it normally remains effective only for that round. However, certain spells do specify special conditions or durations. Be sure to check each spell description carefully.