- All classes begin game with the following common skills, which cost no mana to use:
- Normal Attack (with any weapon excluding throwing potions)
- Throw (with throwing weapons and potions)
- Unsummon (unsummons pets)
- Scroll of Identify (in inventory, identifies items when used)
- Scroll of Town Portal (in inventory, casts town portal when used)
- Assassin and Barbarian alternate between two weapons when using Attack; they both only attack with their right hand when bare-handed and equipping no shield.
- Barbarian can also use Left Hand Throw when equipping throwing weapons or potions.
- Necromancer begins game with a Wand supplying a bonus of +1 to Raise Skeleton (Necromancer Only).
- Sorceress begins game with a Short Staff supplying a bonus of +1 to Fire Bolt (Sorceress Only).
- Many skills use missiles, the properties of which can have a significant impact on their performance.
- Many skills, notably curses and auras, apply their effects within a radius.
Each class also has three of their own skill trees, each containing 10 skills: these skills can be learnt and then used by investing skill points. Characters receive one skill point each time they gain a level-up, and skill points can also be awarded once in each difficulty level for completing the following quests:
A maximum of 20 points can be invested in each skill. Points can only be invested in a skill if points have been invested in its prerequisite skills and if the sum of required skill level and points already invested is less than or equal to character level: while some skills are available at level 1, others only become available at levels 6, 12, 18, 24 and 30. For example, an Amazon cannot invest a point in Freezing Arrow until she is level 30 and she has invested points in its prerequisite skills Cold Arrow and Ice Arrow, and she may not invest another point until she is level 31.
Since 1.13, stat and skill points can be reset as a reward from Akara for cleansing the Den of Evil in each difficulty level, allowing the player to redistribute all points previously allocated. Characters who completed this quest in all difficulty levels before 1.13 will only receive this reward in Hell. In Expansion games, since 1.13 stat and skill points can also be reset by using a Token of Absolution.
Buffs and debuffs active at the time of the reset will remain active until they expire. Native Offensive Auras and Defensive Auras active at the time of the reset will no longer be applied but their graphic may still be displayed, as may their former effects in the character screen. However, this display error stops upon exiting the game and everything returns to normal at the start of the next game. 
Although characters can only invest 20 points in each skill, their level can be increased further by applying various skill bonuses:
- Individual class skill levels (class-specific)
- Individual skill trees (class-specific)
- All Skills
- Fire Skills
- Individual skills (class-specific and oskills allow skills to be used without investing points, if character level >= required level)
Active skills require an action to be taken to use them. All active skills require mana to use, with the exception of the common skills mentioned earlier, Sacrifice, level >12 Magic Arrow and level >8 Double Swing. Skill icons are red if they cannot be used, either due to lack of mana or a required item, an active delay or because it cannot be used in town.
Buffs are active skills which apply a positive effect to the user or to the user and their allies for a duration after being used.
User and allies
Debuffs are active skills which apply a negative effect to the user's enemies for a duration after being used.
- Berserk is the only skill to apply a debuff to the user.
Passive skills require no action to apply their bonuses. Passive skills don't require mana to use, and only apply to the user or pet. Passive bonuses are based on both skill points invested and skill bonuses applied.
Some active skills also apply passive bonuses: Summon Spirit Wolf, Summon Dire Wolf and Summon Grizzly all apply passive bonuses to each other.
Synergy skills apply benefits to other skills based on the number of points invested in the synergy skill. Skill bonuses do not increase synergy bonuses. Most synergy bonuses are mentioned in the skill description, but the following skills also apply hidden bonuses based on the number of points invested in them:
Offensive Auras and Defensive Auras apply positive passive bonuses to the user or to the user and any allies, or negative passive bonuses to any enemies within their radius of effect, as long as the aura is selected on the right mouse button. Auras do not cost mana, with the exception of Prayer.
Oak Sage, Heart of Wolverine and Spirit of Barbs supply auras applying positive passive bonuses to the user and any allies within their radius of effect once summoned.
Using other class skills
Each class can gain access to the skills of another by equipping certain items with the following:
Certain missile weapons with Fires Magic Arrows and Fires Explosive Arrows or Bolts fire those arrows when Normal Attack is used. When a Rogue Scout mercenary is equipped with a bow supplying Fires Explosive Arrows or Bolts, the explosions may be displayed but they apply no damage.