There are many concepts of fatherhood.The question of paternity intertwines three core perspectives on what makes a man a father: while social fatherhood is based on the idea of paternal care for a child, biological fatherhood is rooted in the genetic relationship between a man and the child conceived through his germ cell. Almost perpendicular to this is legal paternity, which links fatherhood to the exercise of parental rights, which can be acquired through legal actions such as marriage, recognition, court determination, or adoption. The aim of this essay is to briefly overview the development of paternity testing within (forensic) medicine since the early modern period. The basic proposition is that the idea of the father has been biomedicalised since early modernity, meaning that paternity has increasingly been expressed in biological and medical terms and that medical experts have gained a crucial role in determining paternity. The text illustrates how closely the concept of biological fatherhood is linked to prevailing theories of conception and (biological) inheritance, and how these theories have shaped forensic methods for establishing paternity. Simultaneously, it is shown how legal development and medical examination interacted as mutually influential factors in the advancement of paternity determination techniques. To highlight these interactions, the text examines the case of Germany where paternity testing became crucial during the Nazi regime and its laws on inheritance. It is argued that paternity was initially invisible and recognisable only through acknowledged sexual intercourse with a woman or marriage on paper; it became visible first through characteristic traits and then retreated into invisibility again, this time in the realm of the molecular, concealed behind abstractions and symbols. The place of evaluation shifted from the courtroom to the laboratory.