Spellcasting class

Some classes have access to spellcasting.

Spellcasting ability
Each of these classes rely on a single "mental" ability to determine the power, quantity and variety of their spells.

In order to gain access to spellcasting of a specific spell level the character must have an equal or higher ability score of 10 + spell level for the class spellcasting associated ability. This means that a spellcasting must have at least 10 in the ability score in order to be able to spellcast at all.

e.g.

For a wizard to gain access to 8th level wizard spells he needs at least 18 intelligence.

Spells known
Sorcerers, bards and assassins have a limited set of spells known derived from their level. This is unaffected by ability score and won't increase from a high score. at level-up, additional new spells are chosen, while currently known spells may also be swapped from new ones.

A wizard begins play with a spellbook containing all 0-level wizard spells (except those from her prohibited spell schools) plus three chosen 1st-level spells. For each point of intelligence bonus the wizard has, the spellbook holds one additional 1st-level spell of your choice. At each new wizard level, she gains two new spells of any spell level or levels that she can cast (based on her new wizard level) for her spellbook. At any time, a wizard can also add spells found in other wizards’ spellbooks to her own.

Clerics, druids, paladins, rangers and blackguards know all spells at the spell levels they have access to.

Spells per day
Spellcasters gain an amount of spells per day they can cast at each spell level derived from their class level.

A high enough ability score will also grant bonus spells to the number of spells which can be cast per day.

Dragon disciple
Dragon disciples gain bonus spells as they gain levels. A bonus spell can be added to any level of spells the disciple already has the ability to cast.

Arcane spell failure
Wizards and sorcerers suffer arcane spell failure when attempting to cast spells with somatic components in any armor other than cloth robes.

Bards and assassins only suffer arcane spell failure when attempting to cast spells with somatic components in armor heavier than light armor.

The remaining classes suffer no arcane spell failure in any armor.

Other benefitial classes
While arcane trickster, harper agent and pale master levels do not directly provide spellcasting abilities, they do enhance the spellcasting abilities of a character's other spellcasting classes.

Warlocks
While the warlock does not have access to spellcasting in the same way as the classes mention, they can create spell-like effects using invocations. This suffer arcane spell failure in armor heavier than light armor.

NWN comparison

 * Assassins and blackguards in NWN did not get a proper spellcasting ability. Instead, they gained a limited selection of spells as spell-like abilities.
 * The indirect spellcasting bonuses from pale master and red dragon disciple levels were not included in NWN.