Over the last fifteen years, increasing numbers of fathers have been attending medical or day-care centers for young children, traditionally "reserved" for mothers and babies. Thus the professionals who work there are able to take an active part in the "co-construction of the fathers" by accompanying their emerging fatherhood. The Research-Action-Training approach used in partnership with the public health service ("Mother and Child Protection" in Paris) has proven to be a particularly interesting tool for modifying the routine found in day-care centers. Through the transfer of knowledge on fatherhood processes and new triadic models, it has led to improvements in the manner fathers are welcome but also babies and the whole family. The preventive approach taken with disturbed father-baby relationships and fatherhood disorders will be illustrated by short clinical examples. (Contains 3 figures and 10 notes.)