Gold nanoparticles (AuNPs) possess distinct optical, chemical, and structural properties that are crucial for various nanotechnology applications. Their size, shape, purity, and dispersity govern their utility in biomedicine, catalysis, and electronics; therefore, precise characterization is essential. Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) provide high-resolution imaging for determining size and morphology. Ultraviolet-visible (UV–Vis) spectroscopy is employed to determine the localized surface plasmon resonance (LSPR) of the nanoparticles, which is useful in evaluating their stability and aggregation behavior. Dynamic light scattering (DLS) measures the hydrodynamic diameter and zeta potential, making it suitable for assessing colloidal stability. X-ray diffraction (XRD) confirms crystallinity, while X-ray photoelectron spectroscopy (XPS) determines elemental composition. This chapter underscores the significance of accurate AuNP sizing and the critical role of these techniques in real-world applications. The advancement of these characterization methods will shape the future of nanotechnology, precision medicine, and sustainable material science.