The ubiquitous contamination of the environment with microplastics (MP), the associated risks to ecosystems and ultimately to human health has recently attracted great public and scientific attention. However, possible risks associated with MP cannot be generalized for several reasons: (1) MP is a collective term for a very heterogeneous class of particles with respect to their size (¡ 1 mm - 5 mm), polymer type, chemical and physical properties as well as surface characteristics. (2) Well-established tools and guidelines for the risk assessment of solutes cannot be transferred to MP as polymers are composed of various chemical compounds, and research on effects of sparingly soluble particulates in the environment is generally scarce. (3) In contrast to macroplastics (¿ 5 mm), there is almost no mechanistic understanding of observed effects yet. (4) Some fundamental deviations between field and laboratory studies (e.g., types and concentrations of MP, selection of exposed organisms, time scales) hamper the understanding of MP effects at the community or ecosystem level. (5) There is a bias in scientific literature towards studies that focus on adverse effects, but for a sound risk assessment, a more balanced view is essential. (6) The interpretation of observed effects of microplastics in the context of measured environmental concentrations and naturally occurring microparticles is often missing. We suggest possible advancements through a careful design of relevant exposure scenarios, use of appropriate reference materials, systematic research on mode of action, and establishing model systems. Furthermore, we emphasize the need for a detailed documentation and communication of experimental parameters for a more balanced and substantial valuation of both effect and non-effect studies.