Door closers are a small piece of hardware with a big job: they control the motion of every door they touch, affecting fire safety, energy efficiency, accessibility, and building security. Yet for such a common device, they are frequently misunderstood. Specifiers buy on price, installers skip the adjustment steps, and owners are left with doors that slam or stick. This handbook fixes that. It covers the mechanical principles, the types and grades, and the specifications that actually matter, all grounded in the reality of the jobsite and the requirements of building codes.