The quest to enlarge the molecular space of functional biomolecules has led to the discovery of selective, mild and high-yielding chemical reactions for the modification of peptides and proteins. These conjugation methods have recently become even more advanced with the advent of modern biochemical techniques such as unnatural protein expression or enzymatic reactions that allow the site-specific modification of proteins. Within this overview, we will highlight recent examples that describe the site-specific functionalization of proteins. These examples go beyond the straightforward attachment of a given functional moiety to the protein backbone by employing either an innovative linker-design or by novel conjugation chemistry, where the modification reaction itself is responsible for the (altered) functional behaviour of the biomolecule. The examples covered herein include ‘turn-on’ probes for cellular imaging with low levels of background fluorescence, branched or cleavable polymer–protein conjugates of high stability within a cellular environment, the installation of natural occurring posttranslational modifications to help understand their role in complex cellular environments and finally the engineering of novel antibody drug conjugates to facilitate target specific drug release.