The many possible causes of poor sperm motility. The chapter covers genetic conditions such as Klinefelter syndrome and congenital absence of the vas deferens, and explains how heat, including from varicocele, may reduce motility by affecting the energy sperm need to move. Infections such as gonorrhoea, chlamydia and epididymitis are discussed alongside the value of regular STI screening. Hormonal imbalances, lifestyle factors like diet, alcohol, smoking and stress, and environmental exposures to pesticides, heavy metals and radiation are also covered. The chapter notes that several factors often combine, and that low sperm count may occur alongside it.