Hundreds of scientific publications now show that microplastics contaminate the world's oceans, yet scientists have only just begun to document and study microplastics in freshwater and terrestrial systems. In this Perspective, Chelsea Rochman emphasizes that, while microplastic contamination has been reported in freshwater animals - including insects, worms, clams, fish, and birds - the effects of this contamination remain largely unknown. Researchers often ask questions about impacts of microplastics in seafood on human health, yet microplastics in dust, groundwater, and agricultural soils may also be of great importance, Rochman notes, citing early evidence that microplastics may travel via soil and atmospheric deposition. She also highlights how studying terrestrial and freshwater systems may be a better means to pinpoint the source of microplastic contamination, compared to the open ocean where particles tend to be weathered, fouled, and highly fragmented. The author concludes that microplastic research must be global and include a greater understanding about the scale, fate, and effects of microplastic pollution at all stages.