The terms "unexplained infertility", or "infertility of unknown causes" can engender a great deal of frustration for infertile couples. There is always a cause for infertility; however, science has not progressed to the point where all etiologies can be identified. Many advances have been made over the last fifteen years and the number of couples falling into the "unexplained infertility" category has declined. As research proceeds, this number is expected to be reduced further. Great strides have been made in the diagnosis and treatment of male infertility and it is now known that subtle abnormalities in sperm quality can lead to sub fertility. This is why the semen analysis is one of the most important components of the infertility evaluation; especially given that male factor is present in up to 40% of infertile couples.