Microplastics are a persistent pollution problem with widespread distribution in the environment, which are known to cause biological and socioeconomic impacts on ecosystems. Assessing microplastic concentrations in environmental matrices is a challenging task, as it requires considering several variables during sampling, processing, and identification of plastics, and also on data reporting. This chapter discusses the diverse range of techniques and methodologies used to assess plastics and microplastics in environmental matrices, by comparing the techniques used in the early 2010s with the current methodologies employed. The main challenges associated with microplastic assessment relate to the lack of standardization processes at a global scale; the lack of an all-inclusive consensus on definitions, and the diverse ways to report results. The versatility and durability of plastic materials allow them to be used in a wide range of technical and technological applications that contributed to advancements in modern life. Nonetheless, its size makes it extremely difficult to retrieve from environmental matrices. Methodologies to assess micro- and nanoplastics will continue to evolve in the next decades, and the adaptation of circular economy approaches to production and disposal of plastics are likely to influence a reduction on the scale of pollution of this material in the environment.