Twenty years after the first publication using the term microplastics, we review current understanding, refine definitions and consider future prospects. Microplastics arise from multiple sources including tires, textiles, cosmetics, paint and the fragmentation of larger items. They are widely distributed throughout the natural environment with evidence of harm at multiple levels of biological organization. They are pervasive in food and drink and have been detected throughout the human body, with emerging evidence of negative effects. Environmental contamination could double by 2040 and widescale harm has been predicted. Public concern is increasing and diverse measures to address microplastics pollution are being considered in international negotiations. Clear evidence on the efficacy of potential solutions is now needed to address the issue and to minimize the risks of unintended consequences.