For more than 20 years, the issue of declining sperm quality in men has remained a subject of debate. The purpose of this review is to provide an overview of long-term trends and regional variations in human sperm characteristics. French studies conducted at the Cecos (Centers for the Study and Preservation of Human Eggs and Sperm) have contributed to this debate: a decline in sperm quality among sperm donors has been observed over time in Paris but not in Toulouse. Of the 33 studies reported in this review, 16 show a secular decline, while five show an increase in sperm characteristics. Regional differences in sperm characteristics have also been identified in eight French centers as well as in other regions of the world. Geographic variations are linked to environmental exposure, as suggested by experimental studies, data from wildlife, and a few studies in humans—though these are still limited in number. Exposure to multiple reprotoxic risk factors—whether environmental or lifestyle-related—is now recognized, necessitating multidisciplinary research. In this context, the authors discuss the value of initiating such research, particularly prospective studies. The national network of Cecos remains a unique tool for facilitating such studies and for establishing a sperm quality observatory in France. Public health measures could result from the findings of such research. (R.A.)