The principles of paternity exclusion and current systems of paternity testing are reviewed. Recent advances, especially the utilization of human leukocyte antigens (HLA), have greatly improved the likelihood of exclusion of the innocent accused male. Failure to exclude an alleged father through multiple test systems increases the suspicion that he is the biological father. This can be expressed mathematically as the Probability of Paternity, for which the principle of calculation is explained. Legal obligations of the laboratory are briefly explored.