Sexual desire, arousal, and orgasm are mediated by complex—and as yet not fully understood—interactions of the somatic and autonomic nervous systems, operating at cerebral, spinal, and peripheral levels. Furthermore, neural activity within these systems is modulated by the presence of steroid and peptide hormones, which affect male and female response differentially. At the central level, dopaminergic and serotonergic systems appear to play a significant role in various components of sexual response, although adrenergic, cholinergic, nitergic, γ-aminobutyric acidergic, and other neuropeptide transmitter systems may contribute as well. At the peripheral level, adrenergic, cholinergic, and nitergic activation mechanisms control vascular changes that underlie vaginal lubrication and penile erection. In addition, these systems respond to descending brain and spinal influences that generate orgasmic response. Disruption of endocrine, neural, or vascular response—caused by aging, disease, surgery, or medication—has the potential to lead to sexual inadequacy. At the same time, psychological and relationship factors play an important role in healthy sexual response and may enhance or impair sexual functioning.