This chapter covers the progress of the colorimetric sensing based on metal nanoparticle, which is experiencing rapid development in analytical chemistry. Colorimetric sensors allow simple, on-site detection of chemical and biological analytes, based on the distinctive optical properties of gold (AuNPs) and silver (AgNPs) nanoparticles. These sensors work based on a localized surface plasmon resonance (LSPR) that causes a color change that can potentially be detected visually by the naked eye, thus increasing accessibility and visibility for non-specialists. Different strategies to enhance the sensitivity and selectivity of these sensors, such as the functionalization of nanoparticles with aptamers, ligands, and polymers provide an insight in this chapter. When coupled with various nanoplasmonic substrates, colorimetric sensing provides an interpretation of color change that can eventually read trace analyte amounts, thus providing new and better diagnostic methods for pollutants, pathogens, and biomarkers for environmental monitoring, food safety, and healthcare applications. In addition, this chapter tries to highlight the practicality of the nanoparticle synthesis processes and their applications in the form of rapid diagnostic kits and portable monitoring tools, providing real solutions to some of real world problems.