Copper nanoparticles (CuNPs) have been extensively utilized as a material for colorimetric sensing due to their size-dependent unique plasmonic properties, low-cost, and catalytic activity. These nanoparticles demonstrate a surface plasmon resonance (SPR) peak in the visible-range of ~ 530 nm that can be finely adjusted by particle size and morphology. Owing to their high surface-to-volume ratio, CuNPs exhibit enhanced interaction with target analytes, which boosts their catalytic activity and amplifies the colorimetric signal, resulting in improved sensitivity. These properties enable their use in diverse applications, including biomolecule detection, environmental monitoring, and medical diagnostics. Still, oxidation and stability are the major challenges that need to be mitigated. The performance and reproducibility of CuNPs can alleviate these challenges by surface functionalization and the fabrication of composite structures. In this chapter, we specifically discuss CuNP-based colorimetric sensors, including their properties, reaction mechanisms, and practical applications, and emphasize their potential in real world applications. Finally, it discusses the remaining challenges and attempts to anticipate the future trends driven at improving the sensitivity, selectivity and stability of such metal nanoparticle sensors.