G-protein-coupled receptors (GPCRs) are the biggest and most diverse group of membrane receptors. They are the targets for almost a third of all approved drugs. Their structural diversity, seven-transmembrane architecture, and capacity to interact with various intracellular signaling partners, including heterotrimeric G-proteins, β-arrestins, and allosteric modulators, render them pivotal to physiological regulation and pharmacological discovery. This chapter offers an extensive examination of GPCR biology, addressing significant historical developments, structural and mechanistic innovations, established and unconventional signaling pathways, and the rise of biased agonism. It looks at modern in vitro and in vivo screening platforms, how specific receptor subtypes are chosen, and the pharmacokinetic–pharmacodynamic factors that affect GPCR ligand behavior. There are detailed sections on safety assessment, toxicology, clinical development strategies, translational challenges, and predictive tools, like systems pharmacology, humanized models, and organ-on-chip technologies. These insights collectively underscore the dynamic progression of GPCR-targeted therapeutics and stress the importance of integrative strategies that improve efficacy, selectivity, and safety while minimizing late-stage clinical attrition.