Skill

Skills are learned abilities of your character. The skill level shows how good your character is at that skill. Skill levels can be increased by allocating skill points earned from gaining levels. Skills can also be improved by reading specific books or completing quests within the games.

In Fallout: Brotherhood of Steel and Fallout 4, "skills" work like perks in other Fallout games. They are not listed here for this reason.

Combat skills


Mechanics
For most skills, the success of an action governed by one is determined by either directly checking skill level against a threshold or with a skill roll.

Rolls are used for most intrinsically random actions like hits in combat and random encounters.

With Speech, direct checks are used as well as rolls in Fallout and Fallout 2, with direct checks completely taking over in later installments, as the randomness the rolls introduce is less desirable in dialogues and quests.

Skills governing the success of gameplay actions with low degree of randomness (like lockpicking) used rolls in classic games, too, but eventually switched to entirely different mechanics (see skill pages for details).

Skill roll
A skill roll (also called "skill challenge") emulates the way skill checks are done in traditional pen-and-paper RPGs.

A random number from 1 to 100 is generated. If it is lower or equal to the skill level, the action is a success. So, for an unmodified check, skill level is effectively a chance of success for a relevant action.

A check can have a modifier that the threshold is adjusted by. Combat skills use automatically calculated modifiers from e.g. the ambient darkness level. Modifiers for checks associated with specific object or logic (like a dialogue option) are set explicitly. So a positive modifier makes the action easier while a negative one - harder. Since skills can grow up to 300%, the hardest challenges can have modifiers of as much as -100.

Some skills also limit the success rate by 95% no matter what.