BIOSID is a Biofidelic Side Impact Dummy which is representative of the fiftieth percentile adult male in size, weight and impact response. The dummy was developed under the direction of the SAE Side Impact Dummy Task Force in response to an International Standards Organization study that concluded that neither the SID nor EUROSID dummy had sufficient lateral impact response biofidelity to be used to assess side impact protection. This paper provides a description of the BIOSID components, the rationale for their selection, and comparisons of its lateral impact responses to the biofidelity requirements specified by the International Standards Organization.