In situ-based effects measures have gained increased acceptance as a means to improve the link between cause and effect in aquatic ecotoxicological studies. These approaches have primarily been employed where more conventional laboratory tests with field collected samples and routine in-field community surveys have failed to provide reasonable answers with respect to causes of toxicity, primary routes of contaminant exposure, and what constitutes ecotoxicologically relevant contaminant levels, at least at a site-specific level. One of the main advantages provided by in situ tests compared to more conventional field-based monitoring approaches is that they provide better control over stressor exposure to a defined population of test animals under natural or near-natural field conditions. In situ techniques can also be used to avoid artifacts related to sampling, transport and storage of contaminated water and sediment intended for laboratory-based toxicity assessment. In short, they can reduce the need for laboratory to field extrapolation and, when conducted properly, in situ tests can provide improved diagnostic ability and high ecological relevance. This paper provides suggestions and considerations for designing in situ studies, choosing test species and test endpoints, avoiding or minimizing test artifacts, best addressing some of the limitations of in situ test techniques, and generally improving the overall quality of the in situ approach chosen.