Amazing advances in software, computer, and network technology occurred during the 1990s. These advances contributed significantly to the increasing impact of models and simulations on numerous scientific and defense-related domains including medical diagnosis, system acquisition and development, planning and analysis, and training. A growing emphasis on processes that ensure that computer models and simulations perform correctly and that appropriate confidence is placed in their results has accompanied this increased use of models and simulations. Verification, validation, and accreditation (VV&A) serve as the cornerstones of simulation correctness and credibility. This article discusses APL's leading role in DoD's VV&A activities and describes the Laboratory's contribution to advances in VV&A methodology.