Click chemistry is a powerful concept that refers to a set of covalent bond-forming reactions with highly favorable properties. In this Perspective, I outline the analogous concept of click biology as a set of reactions derived from the regular building blocks of living cells, rapidly forming covalent bonds to specific partners under cell-friendly conditions. Click biology using protein components employs canonical amino acids and may react close to the diffusion limit, with selectivity in living cells amid thousands of components generated from the same building blocks. I discuss how the criteria for click chemistry can be applied or modified to fit the extra constraints of click biology and achieve favorable characteristics for biological research. Existing reactions that may be described as click biology include split intein reconstitution, spontaneous isopeptide bond formation by SpyTag and SpyCatcher and suicide enzyme reaction with small-molecule ligands (HaloTag and SNAP-tag). I also describe how click biology has created new possibilities in fields including molecular imaging, mechanobiology, vaccines and engineering cellular intelligence.