Exendin-4 is a 39-residue incretin-mimetic peptide from the venom of Heloderma suspectum (Gila monster) and H. horridum (beaded lizard). It is a venom analog of mammalian glucagon-like peptide-1 (GLP-1) and a full agonist towards the GLP-1 receptor to produce insulinotropic effects. The structural properties of exendin-4 explain its better agonistic effect on the GLP-1 receptor as compared with the endogenous GLP-1 peptide. The helical region of the peptide interacts with the extracellular N-terminal domain (NTD) of the GLP-1 receptor, while the C-terminal Trp cage further intensifies its binding to the receptor. Exendin-4 may act as an allosteric modulator to enable in-depth interaction of NTD with the core domain of the GLP-1 receptor, leading to receptor activation. Its promising agonistic effects on the GLP-1 receptor help to preserve normal physiological functions of pancreatic β-cells through anti-apoptotic, antioxidative and anti-inflammatory actions. Moreover, exendin-4 can also attenuate diabetic-related complications such as neuropathy, nephropathy and ventricular remodeling via activation of the GLP-1 receptor. Exendin-4 is more resistant to hydrolysis by dipeptidyl-peptidase 4 and thus, exhibits a longer biological half-life than GLP-1. Therefore, it presents as a preferred therapeutic option for diabetes compared with GLP-1. Furthermore, the neuroprotective properties of exendin-4 have highlighted its potential in the treatment of neurodegenerative diseases, for example, Alzheimer’s disease and Parkinson’s disease. Although exendin-4 has a reasonable subcutaneous bioavailability, its biological half-life is only up to 4 hours, and a repeated dosing regimen is required to maintain its therapeutic levels. Future efforts should be directed toward a different modification approach to improve the pharmacological properties of existing exendin-4. This chapter focuses on an overview of exendin-4 and its related peptides, their pharmacological properties, and their structure–activity relationships with the GLP-1 receptor.