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). Although most hammers are hand tools, powered versions exist; they are known as powered hammers. Types of power hammer include steam hammers and trip hammers, often for heavier uses, such as forging.

​

Some specialised hammers have other names, such as sledgehammer, mallet, and gavel. The term "hammer" also applies to devices that deliver blows, such as the hammer of a firearm, or the hammer of a piano, or the hammer ice scraper.

​

The use of simple hammers dates to around 3,300,000 BCE according to the 2012 find made by Sonia Harmand and Jason Lewis of Stony Brook University, who while excavating a site near Kenya's Lake Turkana discovered a very large deposit of various shaped stones including those used to strike wood, bone, or other stones to break them apart and shape them. Stones attached to sticks with strips of leather or animal sinew were being used as hammers with handles by about 30,000 BCE during the middle of the Paleolithic Stone Age.

​

A traditional hand-held hammer consists of a separate head and a handle, fastened together by means of a special wedge made for the purpose, or by glue, or both. This two-piece design is often used, to combine a dense metallic striking head with a non-metallic mechanical-shock-absorbing handle (to reduce user fatigue from repeated strikes). If wood is used for the handle, it is often hickory or ash, which are tough and long-lasting materials that can dissipate shock waves from the hammer head. Rigid fiberglass resin may be used for the handle; this material does not absorb water or decay, but does not dissipate shock as well as wood.

​

A loose hammer head is hazardous because it can literally "fly off the handle" when in use, becoming a dangerous uncontrolled missile. Wooden handles can often be replaced when worn or damaged; specialised kits are available covering a range of handle sizes and designs, plus special wedges for attachment.

​

Some hammers are one-piece designs made primarily of a single material. A one-piece metallic hammer may optionally have its handle coated or wrapped in a resilient material such as rubber, for improved grip and reduced user fatigue.

​

The hammer head may be surfaced with a variety of materials, including brass, bronze, wood, plastic, rubber, or leather. Some hammers have interchangeable striking surfaces, which can be selected as needed or replaced when worn out.