Microplastics, defined as plastic particles between 1 μm and 5 mm in size, have emerged as a significant environmental concern due to their ubiquitous presence and potential ecological and health impacts. Microplastics are formed either through the direct release of primary microplastics, such as those found in industrial products, or through the fragmentation of larger plastic debris into secondary microplastics due to environmental weathering. This chapter provides a comprehensive introduction to microplastics, covering their definition, origins and generation, and pathways into various ecosystems, as well as the assessment methodologies. Analytical methodologies for assessing microplastics, including sampling, isolation, and identification techniques such as microscopy, spectroscopy, and thermal analysis, are discussed in detail. Furthermore, the key considerations in contamination prevention are listed and discussed. By synthesising current knowledge, the chapter provides a foundational understanding of microplastics for researchers, policymakers, and environmental practitioners, addressing the critical need for informed actions to tackle this global pollutant.