Sexual response refers to the set of physiological and emotional changes that lead to and follow orgasm. Different researchers have constructed various models.

Usually, these models include three, four, or five distinct phases, with the exact components of each phase differing across models.

Helen Singer Kaplan proposed the Triphasic Concept of human sexual response involving three stages: desire, excitement, and orgasm. In his book "Human Sexual Response", Lief described five sexual response phases: desire, arousal, vasocongestion, orgasm, and satisfaction.

William Masters and Virginia Johnson, prominent sex researchers and therapists, suggested that there are four identifiable phases in the sex response cycle: excitement, plateau, orgasm, and resolution.

Using various instruments designed to monitor changes in heart rate and muscle tension, Masters and Johnson were able specify the bodily changes that characterize each of these phases.

The First Phase of Sexual Response
Excitement can last for just a few minutes or extend for several hours. Characteristics of this phase include: an increasing level of muscle tension, a quickened heart rate, flushed skin (or some blotches of redness may occur on the chest and back), hardened or erect nipples, and the onset of vasocongestion, resulting in swelling of the woman's clitoris and labia minora and erection of the man's penis.

Other changes also occur. In the woman, the vaginal walls begin to produce a lubricating liquid, her uterus elevates and grows in size, and her breasts become larger. At the same time, the woman's vagina swells and the muscle that surrounds the vaginal opening, called the pubococygeal muscle, grows tighter.

These changes prepare the woman's body for orgasm and were called the "orgasmic platform" by Masters and Johnson. Additional changes in men include elevation and swelling of the testicles, tightening of the scrotal sac, and secretion of a lubricating liquid by the Cowper's glands.

The Second Phase of Sexual Response
Also known as the plateau, this phase is characterized primarily by the intensification of all of the changes begun during the excitement phase. During this period, the woman's clitoris may become so sensitive that it is painful to the touch. The plateau phase extends to the brink of orgasm, which initiates the reversal of all of the changes begun during the excitement phase.