Proteolysis-targeting chimeras (PROTACs), the hetero-bifunctional compounds containing a specific ligand to bind the target protein, a suitable linker, and an E3 ubiquitin ligase substrate, are being developed for therapeutic applications. PROTACs hijack the catalytic activity of ubiquitin E3 ligases to mediate proteasome dependent degradation of selected protein of interest (POI), by bringing the ligase and POI into close spatial proximity and initiating the poly-ubiquitination process. Compared to the traditional small-molecule drugs, PROTACs reduce the problems of dosage, drug resistance, side effects and undruggable targets that could not be targeted pharmacologically. In this review, all the POIs, and peptide to small-molecule based PROTACs developed during the past two decades are summarized and directions for future development are discussed. © 2020 Elsevier Inc.