Microplastics are matter of debate in environmental science, politics, and the public. They are mostly degradation product of bigger plastic items, abraded fibers from fabrics or fishing gear, or exfoliators from cosmetic products. These particles are accumulating in the marine environment and may be ingested by marine organisms. Apart from microplastics, marine environments and, particularly, coastal regions, contain by nature various microparticles of biological and non-biological origin. These comprise empty or broken diatom frustules, chitinous or calcareous body parts of animals, sand grains, and other minerals. Such natural microparticles can be ingested by marine organisms as well and may cause adverse cellular effects. Vulnerable are especially filter-feeders, and those animals living in estuaries, with constant exposure to a variety of suspended particles. Bigger particles are usually expelled from the digestive system via avoidance, formation of pseudo faeces, or regurgitation. Smaller particles, however, can easily pass the filter systems, enter the digestive organs, and cause cellular stress. A complementary approach of field observations and laboratory experiments will investigate the differences between natural and anthropogenic microparticles in the cellular stress response of the Brown shrimp (Crangon crangon). The aim of this doctoral project is to determine the actual hazard potential of microplastics and define their boundaries for marine decapods.