An Allen wrench, Allen key, hex key or hex head wrench is a tool used to drive screws and bolts with a hexagonal socket in the head.

Some of the advantages of this format of tool are:

• Six contact surfaces between bolt and driver
• Contact surfaces of the bolt are protected from external damage
• The bolt can be inserted into its socket using the key
• Torque applied constrained by the length and thickness of the key
• Very small bolt heads are accommodated
• Can be used with a headless screw

Using a single tool instead of many others reduces tooling costs for the manufacturer and is more convenient for the end user. Many manufacturers have adopted hex key bolts for assembling bicycles or for self-assembly furniture.

"Allen wrench" was originally a trademark of Allen Manufacturing Company in Hartford, Connecticut in 1943. In Europe, it is usually known as an "Unbrako key" (also often misspelled as "Umbrako"), which is a brand name established in 1911. The brand is owned by SPS Technologies.

Note that the image is of a multi-tool, with many Allen keys in one tool. Allen keys are more usually single metal L shaped tools.