<p>Atom transfer radical polymerization (ATRP) is a synthetic method that harnesses radical polymerization to yield well-defined polymers of controlled architecture from a wide range of monomers. It is a form of reversible deactivation radical polymerization, the key step of which is the transfer of a halogen atom between a catalyst and a polymeric radical. ATRP is currently one of the most widely used techniques in macromolecular engineering, providing access to products including block and star copolymers, bioconjugates, nanoparticles and polymer brushes. This Primer aims to give a general introduction to ATRP, describing the basic principles of the mechanism, guidelines for the selection of appropriate catalysts and initiators and typical methods used to check that the desired architecture and functionality have been obtained. A troubleshooting guide suggests solutions to some commonly encountered problems, and some representative applications are reviewed, including reversing ATRP for chemical recycling by depolymerization. The Primer concludes with an overview of the existing limitations of ATRP and optimizations that can address them and an outlook over future challenges and exciting areas of current research.</p>