Around one in four couples will see their gp about infertility problems. Large epidemiological studies show that failure or difficulty in conceiving a child is a surprisingly common, worldwide problem with between one in six and one in seven couples classed as sub-fertile. Sperm dysfunction is the most common cause of infertility and affects one in fifteen men. Studies using semen assessment as the criteria for sub-fertility show that up to one in five 18-year-olds are classed as sub-fertile. Recent reports suggest that the incidence of male infertility is increasing. [With links to the evidence base]