A decrease in sperm counts in the Western world has been observed over the past fifty years and the need for assisted reproduction has been on the rise since the 1990s. An estimated 15% of couples worldwide are considered to be infertile being unable to conceive a child after one year of regular unprotected intercourse. Infertility can be associated with a male or a female factor; however, in 15-20% of the cases, it is idiopathic. The reasons behind these trends are complex but numerous insights suggest that environmental and lifestyle factors play a significant role during fetal life, childhood, and adulthood. The present research has two independent aims that were investigated using different approaches. The first is to evaluate semen quality of young Swiss men and better characterize the potential impact environmental or lifestyle factors might have on reproductive health using an epidemiological approach. The second aim is to investigate the adverse effect of pharmaceutical drugs on human sperm fertilizing capacities.