Since the first introduction of their concept in the 1980s and 90s, polymer brushes have been the focus of intense research efforts to identify novel physico-chemical properties and responsiveness, and optimise the properties of associated interfaces for an ever growing range of applications. To a large extent, this effort has been enabled by progress in surface initiated controlled polymerisation techniques, allowing a huge diversity of monomers and macromolecular architectures to be harnessed and achieved. However, polymer functionalisation through chemical coupling of various moieties and molecular structures has also played an important role in expanding the molecular design toolbox of the field of polymer brush science. This perspective article reviews recent progress in polymer brush functionalisation, discussing a broad range of strategies for the side chain and end chain chemical modification of these polymer coatings. The impact of the brush architecture on associated coupling is also examined. In turn, the role that such functionalisation approaches play in the patterning and structuring of brushes, as well as their conjugation with biomacromolecules for the design of biofunctional interfaces is then reviewed and discussed.