With the explosive growth of nanotechnology, questions have arisen whether the use of new nanoscale materials might have unintended human health and environmental consequences. Studies of biological effects of nanoscale materials that might answer these questions have lagged behind other aspects of nanotechnology development. There is widespread interest in addressing this information need, but conducting studies of biological interactions of nanoscale materials poses several challenges. Issues regarding safe handling of nanoscale materials by researchers have not been well worked out. Also, characterization of test materials is more complex than with conventional studies of chemicals because it involves, in addition to composition, aspects of size, shape, and surface properties. There are questions of proper dosimetry and potential problems in detecting some nanostructures in biological tissues. As investigation of biological effects of nanostructures moves forward, it will be important to reach consensus on requirements for safe use of nanomaterials, characterization of test materials needed such that studies can be correctly interpreted and reproduced, and ways in which experimental designs should be modified to address special issues associated with nanostructures. Through this workshop, investigators in the field will lay the foundation for developing a set of best experimental practices for studies of potential toxicity of nanoscale materials.