Gold nanoparticles (AuNPs) are nanoscale particles of gold with distinct physical, optical, and chemical properties that are crucial for nanotechnology. Localized surface plasmon resonance (LSPR) is a phenomenon of collective electron oscillation in noble metals, and offers single-molecule detection, hence, they find their application in biosensing, imaging, and photothermal therapy. The optical behavior is a function of size, shape, and the surrounding medium of the AuNPs, which can be used to tune the plasmonic response for specific applications. Chemical functionalization is performed to improve AuNP stability, selectivity, and reactivity. Surface enhancements, such as ligand replacement and bioattachment, are applied to improve biocompatibility and targeting for drug delivery, catalysis, and biosensing. The functionalized AuNPs have been applied in colorimetric sensing, cancer therapy, and environmental monitoring. However, the toxicity of the nanoparticles is an issue that depends on size, surface charge, and coatings of the particles. These risk factors are reduced through AuNP surface engineering, to enhance the use of AuNPs in medicine and industry. In this chapter, basic principles and functionalization strategies of AuNPs, and their major areas of application are reviewed with an emphasis on the role of these nanoparticles in nanomedicine, diagnostics, and sustainable emission.