Several inherited conditions associated with testicular defects, abnormal spermatogenesis and morphologically abnormal sperm have been found. These usually are controlled by single gene pairs. A notable exception is testicular size, with heritability in young bulls ranging from .42 to .88. Testicular size directly affects sperm output potential. The major contributor to variation in semen quality is the environment. Environmental effects may be temporary or permanent. Permanent effects occurring during prenatal and prepubertal periods and temporary or permanent factors acting after spermatogenesis is initiated can alter semen quality. Semen quality improves during the first few months after puberty and declines in old age. Malnutrition and the ingestion of toxic materials can have a major effect on testicular development and spermatogenesis, but the reproductive system has considerable regenerative capacity unless the dietary deficiencies are severe and prolonged. Elevated testicular temperatures resulting from incomplete descent of the testes (cryptorchidism), high environmental temperatures or inflammation are detrimental to spermatogenesis in all scrotal mammals. Cold temperatures appear to be innocuous unless actual freezing of tissue occurs. During periods of decreasing daylight semen quality declines in stallions and improves in seasonally breeding sheep. The time required to form and transport sperm in bulls, rams, boars and stallions is about 64, 56, 47 and 59 days. Therefore, a considerable potential lag may exist between a testicular event responsible for a change in semen quality and the time that this change is evident in ejaculated semen. Conditions imposed at the time of semen collection such as frequency of ejaculation, degree of sexual preparation and type of semen collection may influence the quality of semen harvested. Finally, certain semen characteristics are more variable than others and investigators should utilize suitable existing data to design the most effective least-cost experiments.