Throughout the history of medicine, there have been difficulties in forensic proving of unsolved paternities, making this a major andrological-forensic challenge. In the Roman empire, children procreated by incest, called 'incestuosi', had no rights at all and other illegitimates, e.g. children born of prostitutes or as a result of adultery, could not assert rights towards the father. Only through the 'corpus iuris civilis' of the Roman Emperor Justinian I (6th century ad) could claims of alimony be made in some cases. In later times, evidence of presumable impotence of a potential father was determined by the congressus, where defined juristical 'sexual fights' had to be held before official matrons, surgeons and priests. Men had to demonstrate their erectile and ejaculatory function. A special significance must be attributed to the microscopical discovery of spermatozoa by the medical student Johann Ham in 1677 and the further investigation by Antoni van Leeuwenhoek. After centuries of personally biased and scientifically incorrect concepts of paternity determination, the discovery of the blood grouping system by Karl Landsteiner at the beginning of the 20th century paved the way for sophisticated techniques to prove or rather to disprove paternity. In 1921, Reuben Ottenberg from New York was the first to suggest the application of the system to medico-legal questions, but it was not before the 1950s that legal courts accepted this method. A famous example of disregarding clear blood grouping results was the case of the movie star Charles Chaplin in California in 1946. Modern DNA techniques, today the golden standard in forensic medicine, have also been applied recently for historical investigations. One of the very illustrious examples is the dispute about Thomas Jefferson, third President of the US, fathering a child of one of his slaves.