Metal nanoparticles (NPs) are important building blocks for colorimetric sensors, offering high sensitivity, rapid response, and wide application in biosensing, environmental monitoring and medical diagnostics. The specific properties of metal NPs, such as optical, electronic and catalytic properties, are reviewed to demonstrate their role in enhancing detection methods for various analytes like DNA, RNA, proteins, heavy metals and toxins. The integration of metal NPs into biosensors has led to significant improvements, resulting in more efficient detection of biomarkers, environmental pollutants, and pathogens. Key applications include the development of portable and cost-effective diagnostic devices, real time environmental monitoring systems, and food safety sensors. Moreover, the chapter highlights the role of hybrid and composite metal NPs in enhancing the sensitivity and selectivity of sensors. Metal nanoparticle-based colorimetric sensors are thus expected to transform diagnostics, environmental monitoring, and quality control within the biomedical and food industries, addressing critical global health and safety challenges.