This review essay focuses on the extent to which and in what areas physical evidence influences the processing of defendants through the system. Laboratory or "technical" concerns and probability or statistical concerns emerge as two types of operational issues in DNA evidence testing and relate to some current legal issues. Legal issues associated with DNA testing include invasiveness, the standards of proof, the probative value of the evidence, privacy concerns, and admissibility. Many DNA experts, including some who serve as expert witnesses in trials, are particularly disturbed about the unanswered problems of reliability and validity. Most of the estimates of identical matches are very high, but it is difficult to assess the accuracy of those estimates and to know against what population base to compare the estimates. Both proponents and skeptics of DNA identification agree that setting a national standard for laboratory techniques and certifying laboratory personnel would improve genetic techniques. 2 footnotes and 38 references