PIP: A large body of research confirms the vulnerable nature of spermatogenesis to relatively small increases in testicular temperature. Other physical properties of electromagnetic and ultrasound waves have additive or synergistic effects to those of heat and allow disruption of spermatogenic processes at minimal temperature elevations. In addition, there is a rebound in sperm count following heat-induced suppression of spermatogenesis. These findings suggest the theoretical viability of testicular heating as a reversible method of male contraception in humans. However, before heating techniques can be used for male fertility control, several questions remain to be investigated. The lowest effective doses and the lowest frequency of application of each method of testicular heating necessary for inducing and maintaining a reliable infertile state must be established. It must be determined whether long-term exposure leads to permanent damage or compromise of testicular elements or functions, and whether exposure to simple heat, electromagnetic waves, or ultrasound induces significant changes in the biological constituents of human semen. Also unclear is the exact mutagenic potential of thermal agents on the human gonads. Another research question is whether scrotal warm sensory input rises during testicular heating to a level that is sufficient to alter body core temperature. Rapidly advancing knowledge of biologic constituents of human semen and the increasingly available detection methods of these constituents will enhance research in these areas. It is through such research that the safety, efficacy, and applicability of thermal manipulation of spermatogenesis as a method of male contraception will be established or refuted.