Retaining glycosidases employ a two-step double displacement mechanism to hydrolyze their substrate glycosides. This mechanism involves a covalent enzyme-substrate adduct, and irreversible retaining glycosidase inhibitors have been designed based on this mechanism. Tagging such inhibitors with a reported moiety (biotin, fluorophore, bioorthogonal tag) provides activity-based retaining glycosidase probes. This chapter describes research on such activity-based probes that are inspired by the natural product retaining β-glucosidase inhibitor, cyclophellitol. Modulation of the configuration and substitution pattern yielded a suite of probes with which a host of retaining glycosidases are inhibited, and reported on, including enzymes involved in human pathologies (cancer, inherited lysosomal storage disorders). This chapter provides insights into their design and synthesis, their application in disease diagnosis, and their application in drug discovery, both as tools to uncover competitive inhibitors and as starting point for the design of covalent inhibitors.