Glycogen Phosphorylase (GP) is an important target for the development of anti-hyperglycaemic drugs. GP is an enzyme which is moderated allosterically with multiple ligand binding sites where inhibitors can potentially modulate enzyme activity. The search for potent and isoform selective inhibitors of GP is ongoing with an increasing focus on allosteric inhibition. In this review, the structural diversity, and enzyme interactions of the most recent inhibitors, and in particular allosteric inhibitors, of GP at the different key binding sites is explored. A range of inhibitors of GP, with known as well as unknown binding site or mechanism is presented.