Overall, there are some conclusions that may be made. The biological activity of microplastics depends on particle size, with smaller particles evincing larger effects. Refinement of analytical tools to detect and quantify smaller microplastic particles (nanoplastics) is thus important to clarify the exposures and effects of microplastics. From known observations, however, microplastics are ubiquitous, with water, salt, foods, soil, and even the air containing them (beyond occupational exposure). The shapes and surface functionalization of particles are likely important in determining protein and bacterial binding to microplastics, their uptake, and their toxicities. The uptake of microplastics is not completely defined, but uptake of smaller microplastics through the skin and by ingestion appears to be higher than the uptake of larger microplastics, and the shape (particles versus fibers) of microplastics is also likely to alter their retention. Microplastics appear to cause inflammation (particularly gut inflammation), reduced intestinal mucus, and perturbation of the intestinal microbiome; because of their size, microplastics are treated as foreign bodies rather than being detoxified as a chemical agent. They also alter lipid metabolism and inhibit acetylcholinesterase. Microplastics may act as estrogenic agents, either directly or through additives or substances present in the plastic particles.