A hammer is a tool that delivers a blow (a sudden impact) to an object. Most hammers are hand tools used to drive nails, fit parts, forge metal, and break apart objects. Hammers vary in shape, size, and structure, depending on their purposes.

Hammers are basic tools in many trades. The usual features are a head (most often made of steel) and a handle (also called a helve or haft). Most hammers are hand tools, but there are also many powered versions, called power hammers (such as steam hammers and trip hammers) for heavier uses, such as forging.