Medical genetics was a low-ranked speciality in France until the late 1980s. A few technologies could be mobilised to evaluate genetic risk and conduct counselling sessions. Chromosome analysis and statistical surveys of defects in newborns were combined with routine clinical examination. Although medical geneticists were usually pediatricians, they were viewed by many clinicians acting in the wards as resources that could be employed to manage the uncertainty and failure of clinical work and cope with the anxiety of families. The spreading of tools and instruments developed by molecular biologists as well as the establishment of large research programmes on the analysis of the human genome have triggered important changes. Medical genetics is now viewed by the general public as a highly scientific and promising field, Polymerise chain reaction (PCR) kits, DNA probes or sequencing machines are elements of a new form of life which complement the description of symptoms and the analysis of pedigrees. Specialists in oncology, haematology, cardiology, etc. are eager to launch collaborations with medical geneticists. This chapter will address these changes by focusing on one setting, the medical genetics department of a sick children’s hospital in a French metropolis. It will discuss the local courses of actions which combine genotypes and symptoms, narrow diagnostic uncertainties, and stabilise the bonds between genetic research and medical work.