ABSTRACT. – Meiofauna have been used in numerous experiments to assess pollutant effects and to establish standards for water, soil and sediment quality. Many meiofauna can be reared in relatively easy-to-maintain cultures, and over 50 species have been used in laboratory-based bioassay experiments. Meiofauna species may be exceedingly sensitive to pollutants; reproduction and development are typically much more sensitive than adult survival. Sublethal effects of pollutants are becoming commonly studied with meiofauna because meiofauna lend themselves well to experiments that measure ingestion rate, and pollutant effects on fecundity and population density can be predicted from whole-life-cycle experiments with demographic techniques. Toxic responses of sediment-associated meiofauna generally reflect sediment and porewater chemistry. However, some meiofauna may form cryptic species complexes, and differences in tolerance within species and among cryptic species are possible, increasing uncertainty in toxicity tests. A model nematode species (i.e., Caenorhabditis elegans) and various model harpacticoid copepods (e.g., tigriopus, tisbe, amphiascus) have been identified, and are increasingly being used in association with a growing baseline of biological, genetic and ecological data to elucidate the mechanisms of toxicity. Meiofauna have also been used in studies that determine acclimatory and genetic responses to contaminant exposure, and that examine gene expression profiles from contaminated environments. Field and microcosm experiments with hazardous substances suggest that meiofauna have many poorly understood species interactions with primary producers, predators and competitors that are manifested as indirect, ecological effects in the presence of contaminants. The continued use of meiofauna in assessment studies for newly discovered environmental concerns is justified.