A ladder is a vertical or inclined set of rungs or steps commonly used for climbing or descending. There are two types: rigid ladders that are self-supporting or that may be leaned against a vertical surface such as a wall, and flexible ladders, such as those made of rope or wire rope, that may be hung from the top.

The vertical members of a rigid ladder are called "stringers" or "rails" (US) or "stiles" (UK). Rigid ladders are usually portable, but some types are permanently fixed to a structure, building, or equipment. They are commonly made of metal, wood, or fiberglass, but they have been known to be made of tough plastic.