Areas where use of DNA profiling evidence may provide opportunities for challenge in court include: human error, contamination and degradation, interpreting the DNA evidence, standards for declaring matches, and statistical interpretation. The article also discusses the broader areas of extracting and comparing DNA samples and using and presenting DNA evidence. The article presents 13 procedures that should be followed where DNA evidence is involved, including: (1) the scientist should adduce the evidence of DNA comparisons between the crime stain and the defendant's sample together with calculations of the random occurrence ratio; (2) the Forensic Science Service should make available to a defense expert, if requested, the databases upon which the calculations were founded; (3) the expert should give the jury the random occurrence ratio--the frequency with which the matching DNA characteristics are likely to be found in the population at large; and (4) the summation by the trial judge should include careful directions with respect to any issues of expert evidence. Notes