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A screwdriver is a device used to insert and tighten, or to loosen and remove, screws. There are many varieties, of which the most common are the flathead, and the Phillips, PoziDriv, and SupaDriv heads (crosspoint).

Screwdrivers come in a large variety of sizes to match those of screws, from tiny jeweller's screwdrivers up. It is important to use a screwdriver that is the right size and type for the screw used, or it is likely that the screw will be damaged in the process of tightening it. This is less important for PoziDriv and SupaDriv, which are designed specifically to be more tolerant of size mismatch. When tightening a screw with force, it is important to press the head hard into the screw, again to avoid damaging the screw.

Some manual screwdrivers have a ratchet action whereby the screwdriver blade is locked to the handle for clockwise rotation, but uncoupled for counterclockwise rotation when set for tightening screws; and vice versa for loosening.

Many screwdriver designs have a handle with detachable head (the part of the screwdriver which engages with the screw), allowing a set of one handle and several heads to be used for a variety of screw sizes and types. This kind of design has allowed the development of electrically-powered screwdrivers, which as the name suggests use an electric motor to rotate the bit. Some drills can also be fitted with screwdriver heads. Manual screw drivers with a spiral ratchet mechanism to turn pressure into rotational motion also exist, and predate electric screwdrivers.

Many recent electrical appliances, if they use screws at all, use screws with heads of non-standard shape in an attempt to prevent users of the device from easily disassembling them— see the article on screws for details. Torx is one such pattern which has become very widespread, with suitable screwdrivers widely available in electronics stores. Other patterns of security screws are less common, though sets of security heads are, again, readily available.