G protein-coupled receptors (GPCRs) are the largest family of cell surface receptors and are the primary target for chemical communication via hormones, peptides, neurotransmitters, and environmental stimulants. GPCRs participate directly or indirectly in most (patho)-physiological signaling processes making them a major class of drug targets as evidenced by accounting for more than 30% of marketed drugs. GPCR signaling is intrinsically an allosteric process as it involves the communication between spatially distinct yet conformationally linked binding sites across the cell membrane. Furthermore, it is now well established that most GPCRs have allosteric binding sites that are conformationally linked yet spatially distinct from the endogenous ligand binding site. Designing ligands to specifically target these allosteric sites opens up the exciting opportunity to develop drugs that can be selective towards a specific GPCR subtype and/or fine-tune the activity of the endogenous ligands, akin to the action of dimmer switch. Here I will present recent structural studies from our group utilizing X-ray crystallography and cryo-EM to investigate how allosteric modulators bind to and modulate the function of muscarinic acetylcholine receptors and the adenosine A1 receptor. Together with previous studies, this work will highlight the wide diversity of druggable allosteric sites across GPCRs.