There are multiple types of plastic polymers in the marine environment. Understanding the composition of polymers offers scientists valuable insights into its physicochemical properties and degradation process. The environmental deterioration of plastics plays an important role in the aquatic ecosystem in the creation, dispersion, and buildup of microplastics. Therefore, this chapter provides a concise overview of the processes by which synthetic polymers break down in the environment, several procedures for sampling water and sediment, as well as sample treatments. These treatments include separating plastic particles, which is essential before identifying microplastics. Furthermore, this chapter presents the methodologies employed to determine the chemical composition of plastic particles. The approach outlined in this chapter successfully identified microplastics in soil and water samples taken from estuaries, beaches, and the sea in India.