Gold nanoparticles (AuNPs) are one of the most versatile and accessible classes of nanomaterials. Their chemical stability, ease of colloidal synthesis, surface functionalization, and plasmonic resonance—tunable from the visible through the near-infrared—have made AuNPs the metal nanoparticle of choice for many applications. This article summarizes the chemical synthesis of AuNPs, particularly gold nanorods, with a focus on recent developments in shape control and surface modifications. Current applications using the photothermal properties of AuNPs, as well as AuNP connections to biology and the environmental sciences, will be discussed.