Colorimetric sensors based on gold nanoparticles (AuNP) have attracted great attention in analytical chemistry due to their extraordinary sensitivity and versatility. This chapter explores quasi-two-dimensional nanoparticles and metallic nanoparticles, highlighting how their characteristic features such as size, shape, and low extinction coefficients contribute to their localized surface plasmon resonance (LSPR) optical properties in response to light. These unique properties allow AuNPs to sense trace amounts of analytes, where colorimetric changes act as a visible signal. However, several challenges such as nanoparticle agglomeration, environmental sensitivity, and limited quantitative detection still exist. To improve stability, selectivity, and accuracy of these AuNPs, surface functionalization with stabilizing agents such as polyethylene glycol (PEG) polymers and thiol groups has been suggested. This chapter goes through the different reaction mechanisms of AuNP-based colorimetric sensors, with applications for environmental monitoring, medical diagnostics, and food safety. Moreover, this chapter also aims to discuss sensor efficiency improvement strategies, including both aggregation-based and non-aggregation-based detection approaches. Despite these current limitations, the use of AuNP-based sensors is poised to promote real-time, on-site detection of targeted analytes in various applications.