Targeted protein degradation has matured from a laboratory curiosity into a clinically validated therapeutic strategy, and two chemical modalities now define its practice: monovalent molecular glue degraders and heterobifunctional proteolysis-targeting chimeras (PROTACs). Both redirect the cell's own ubiquitin-proteasome system to eliminate a disease-driving protein rather than merely occupy it, yet they arrive at that shared endpoint by opposite chemical logics. A PROTAC tethers a target ligand to an E3 ligase recruiter through a linker and builds a ternary complex from two independent binding events. A molecular glue is a single small molecule that remodels the surface of an E3 ligase so that a new protein-protein interface forms with a neosubstrate that neither partner would otherwise recognise. This review sets the two modalities side by side across four axes that decide their fate in drug development: mechanism and the thermodynamics of induced proximity, physicochemical property space and its consequences for oral drug-likeness, the origins of degradation selectivity, and the state of clinical translation as of 2026, the year the first PROTAC reached the market. The argument advanced here is that glues and PROTACs are not competitors so much as complementary answers to different starting problems, that the boundary between them is becoming porous as cooperative PROTACs behave increasingly like intramolecular glues, and that the central near-term constraint on both is the narrowness of the exploited E3 ligase repertoire rather than any ceiling on target scope.