Foreskin
The foreskin or prepuce (a technically broader term that also includes the clitoral hood, the homologous structure in women) is a retractable double-layered fold of skin and mucous membrane that covers the glans penis and protects the urinary meatus when the penis is not erect. Almost all mammals have foreskins, although in these non-human cases the foreskin is usually a sheath into which the whole penis is retracted. Only monotremes (the platypus and the echidna) lack foreskins.

# The human foreskin

## Description
In humans, the outside of the foreskin is like the skin on the shaft of the penis but the inner foreskin is a mucous membrane like the inside of the eyelid or the mouth. Like the eyelid, the foreskin is free to move.  Smooth muscle fibres keep it close to the glans but make it highly elastic. The foreskin is attached to the glans with a frenulum which helps retract the foreskin over the glans. At the end of foreskin there is a band of tissue called the ridged band which, according to one study, is rich in nerve endings called Meissner's corpuscles. According to a NOCIRC-funded study by Sorrells et al., the five most sensitive areas of the penis are on the foreskin.
In children, the foreskin normally covers the glans completely but in adults this need not be so. Schoberlein  found that about 50% of young men had full coverage of the glans, 42% had partial coverage, and in the remaining 8%, the glans was uncovered. After adjusting for circumcision, he stated that in 4% of the young men the foreskin had spontaneously atrophied (shrunk). See also : foreskin coverage index

## Development
Eight weeks after fertilization, the foreskin begins to grow over the head of the penis, covering it completely by 16 weeks. At this stage the foreskin and glans share an epithilium (mucous layer) that fuses the two together. It remains this way until the foreskin separates from the glans.
At birth, the foreskin is usually still fused with the glans. As childhood progresses the foreskin and the glans gradually separate, a process that may not be complete until the age of 17. Thorvaldsen and Meyhoff reported that average age of first foreskin retraction in Denmark is 10.4 years.  Wright argues that forcible retraction of the foreskin should be avoided and that the child himself should be the first one to retract his own foreskin. Premature retraction may be painful, and may result in infection.

## Functions
Some researchers believe that the foreskin facilitates intercourse.  In her book Sex as Nature Intended It, Kristen O'Hara argues that foreskin is a natural gliding stimulator of the vaginal walls during intercourse, increasing a woman's overall clitoral stimulation and helping a woman achieve orgasm more often and more quickly.   She therefore believes that the absence of the foreskin and gliding action makes it more difficult for a woman to achieve orgasm during intercourse.
Taylor et al described the foreskin in detail, documenting a ridged band of mucosal tissue. They stated "This ridged band contains more Meissner's corpuscles than does the smooth mucosa and exhibits features of specialized sensory mucosa." The AAP noted that the work of Taylor et al "suggests that there may be a concentration of specialized sensory cells in specific ridged areas of the foreskin." In 1999, Cold and Taylor stated "The prepuce is primary, erogenous tissue necessary for normal sexual function." Moses and Bailey (1998}, however, describe the evidence as "indirect," and state that "aside from anecdotal reports, it has not been demonstrated that this is associated with increased male sexual pleasure."
Gairdner (1949) states that the foreskin protects the glans but some studies show that inflammation of the glans is more common when the foreskin is present.
Shen (China) found a statistically significant *(p = 0.001) increase in erectile dysfunction following circumcision. Pang and Kim (South Korea) reported "Of those who were circumcised long after they had been sexually active, > 80% reported no noticeable difference in sexuality, but a man was twice as likely to have experienced diminished sexuality than improved sexuality." In another study by Kim and Pang (2006) of 255 men circumcised after the age of 20 and 118 who were not circumcised, they reported that masturbatory pleasure decreased in 48% of the respondents and increased in 8%. Masturbatory difficulty increased in 63% but was easier in 37%. 20% reported that their sex life was worse after circumcision and 6% reported that it had improved (the abstract is silent about the other 74%). "There were no significant differences in sexual drive, erection, ejaculation, and ejaculation latency time between circumcised and uncircumcised men." They concluded, "There was a decrease in masturbatory pleasure and sexual enjoyment after circumcision, indicating that adult circumcision adversely affects sexual function in many men, possibly because of complications of the surgery and a loss of nerve endings."   Sorrells et al. (2007), in a study funded by NOCIRC, compared penile sensitivity in 91 circumcised and 68 uncircumcised men and concluded, "The transitional region from the external to the internal prepuce is the most sensitive region of the uncircumcised penis and more sensitive than the most sensitive region of the circumcised penis."
Fink's study of American men also found significantly worsened erectile function *(p = 0.01) Other studies came to different conclusions. Collins (USA), Senkul (Turkey), and Masood (Britain) found no significant difference in erectile function. Senkul found that the circumcised men took significantly longer to ejaculate after circumcision *(P = 0.02). Laumann's study of American-born men found "little difference between circumcision status and sexual dysfunction for the two younger cohorts" (18-29 and 30-44). However, older men (45-59) with foreskins in his sample were significantly more likely to suffer from erectile dysfunction overall *(p < 0.05) and trouble achieving and maintaining an erection *(p. < 0.05). Premature ejaculation and performance anxiety were also noted *(both p. < 0.10). Circumcision rates were also significantly different in different ethnic groups (less common in Blacks and Hispanics) and they varied with the education level of the mother (less common in those with less education).
Fink's study reported less sensitivity after circumcision, though this only bordered on statistical significance *(p = 0.08). In contrast, Masood et al. reported improved sensation in 38% of men following circumcision and less sensation in 18%. 61% expressed greater satisfaction following removal of the foreskin, less satisfaction in 17%, and no change in 22%.
Interpretation of these findings vary. For example, Masood said, "Penile sensitivity had variable outcomes after circumcision. The poor outcome of circumcision considered by overall satisfaction rates suggests that when we circumcise men, these outcome data should be discussed during the informed consent process." Hill and Denniston listed Senkul's finding of an increased ejaculatory time as a "demonstrated adverse effect" of circumcision However, Senkul stated: "Adult circumcision does not adversely affect sexual function. The increase in the ejaculatory latency time can be considered an advantage rather than a complication. However, concerning the cause of that increase, in a Muslim community, the psychological influence of circumcision may be more pronounced than the organic effect."
Some do not accept that the presence or absence of the foreskin makes any difference and as such has no sexual effect.
The fold of the prepuce maintains sub-preputial wetness, which mixes with exfoliated skin to form smegma. Some authors believe that smegma contains antibacterial enzymes, though their theory has been challenged. Inferior hygiene has been associated with balanitis, though excessive washing can cause non-specific dermatatis.
The term 'gliding action' is used in some papers to describe the way the foreskin moves during sexual intercourse. A foreskin that covers the glans penis may move back and forth over the glans.  This gliding movement may reduce friction during sexual intercourse.  The gliding action was described by Lakshamanan & Prakash in 1980 The outer layer of the prepuce in common with the skin of the shaft of the penis glides freely in a to and fro fashion... 
Several genital integrity activists have argued that the gliding movement of the foreskin is important  during sexual intercourse:
- Warren & Bigelow claim that gliding action would help to reduce vaginal dryness and that restoration of the gliding action is an important advantage of foreskin restoration. 
- A survey by Bensley & Boyle provides some confirmation that gliding action provides protection of vaginal lubrication. The authors explain, however, that their subjects were self-selected and a larger sample size is needed.
- O'Hara describes the gliding action:
- Fleiss and Hodges claim: The foreskin's double-layered sheath enables the penile shaft skin to glide back and forth over the penile shaft. (p.24) and The foreskin enables the penis to slip in and out of the vagina nonabrasively inside its own slick sheath of self-lubricating movable skin. (p.26)
- Taylor suggests that the gliding action, where it occurs, may stimulate the nerves of the ridged band , and speculates that the stretching of the frenulum by the rearward gliding action during penetration triggers ejaculation. 
Taves used a single subject to test the actual force required to penetrate a measuring apparatus.  When the foreskin was retracted a more than  tenfold increase in force was needed.  He argued that this confirms the belief of Morgan (1967) that the foreskin makes sexual penetration easier during sexual intercourse. Whiddon (1953) and Foley (1966) also believed that the presence of the foreskin made sexual penetration easier  
A 2002 study into changing circumcision practices in Tanzania Africa, found that there was a significant move towards popularizing circumcision, mostly for perceived health reasons, but that participants (unclear whether male focus groups or female as well) reported it also led to improved sexual pleasure for men and women alike. Some participants in this context, compared the presence of a foreskin to a condom.

## Conditions
Frenulum breve is where the frenulum is insufficiently long to allow the foreskin to fully retract, which may lead to discomfort during intercourse. The frenulum may also tear during intercourse. Phimosis is a condition when the foreskin of an adult cannot be retracted properly. (Before adulthood, the foreskin may still be separating from the glans.)  Phimosis can be treated by gently stretching the foreskin, by changing masturbation habits, using topical steroid ointments, preputioplasty, or by circumcision.  See phimosis for more information.
A condition called paraphimosis may occur if a tight foreskin becomes trapped behind the glans and swells as a restrictive ring. This can cut off the blood supply, resulting in ischaemia of the glans penis.
Aposthia is a rare condition in which the foreskin is not present at birth.

## Surgical and other modifications of the foreskin
Circumcision is the removal of the foreskin, either partially or completely. It may be done for religious, aesthetic, health, or hygiene reasons, or to treat disease.
Preputioplasty is a procedure to relieve a tight foreskin without resorting to circumcision.
Other practices include genital piercings involving the foreskin and slitting the foreskin.

## Research use
Foreskins obtained from circumcision procedures are frequently used by biochemical and micro-anatomical researchers to study the structure and proteins of human skin. In particular, foreskins obtained from newborns have been found to be useful in the manufacturing of more human skin.

## Langerhans cells
Langerhans cells are immature dendritic cells that are found in all areas of the penile epitelium, but are most superficial in the inner surface of the foreskin.   The recent Szabo and Short (2000) study targets Langerhans cells as receptors of HIV, and states that these cells "must be regarded as the most probable sites for viral entry in primary HIV infection in men." Langerhans cells are also known to express the c-type lectin langerin, which may play a role in transmission of HIV to nearby lymph nodes. However, de Witte et al. (2007) reported that langerin, produced by Langerhans cells, blocks the transmission of HIV.

# Foreskin in non-human species
In koalas the foreskin contains naturally occurring bacteria that play an important role in fertilization.