**Abstract**

Due to the heterogeneous nature of the physiochemical properties of microplastics (MPs), their behaviour in the environment is quite complex compared to other contaminants. The variety of polymers, wide range of sizes, variable shapes and numerous colours influence their mobility, transport and distribution in the different environmental compartments. For example, different shapes and sizes are distributed differently, which influence their bioavailability and ecological impacts. The uptake of MPs by aquatic biota also depends, among others, on their characteristics. This book chapter aims to discuss the ecological and toxicological impacts of MPs in relation to their physical and chemical properties. The chapter starts with a brief introduction explaining the uniqueness of MPs as emerging contaminants and a driver of environmental change. The following two sections then provide deeper insights into their ecological impact at all levels of the ecosystem and highlight the complexity associated with their toxicological effects. Finally, the last section provides more discussion about their properties in the context of their environmental behaviour, fate, bioavailability and toxicity.

**Keywords:** microplastics, physiochemical properties, behaviour, bioavailability, toxicity
