In mammals, the foreskin or prepuce is the retractable double-layered fold of skin and mucous membrane that covers the head of the penis, (the glans penis). It serves as a sheath to protect the glans penis. In a female, the clitoral foreskin, or hood, covers and protects the glans clitoris in a similar manner.
The human foreskin
In humans, the foreskin, a part of the penis, is a double-folded tube of skin and mucous membrane (similar to the inside of the mouth) which attaches at the crested end of the glans at the sulcus, and to the body above the pubic bone and scrotum. Whereas the skin on most of the body is attached to the underlying tissue, the foreskin, like the eyelid, is free to move. Smooth muscle fibres keep the foreskin close to the glans penis but make it highly elastic.
The inner surface of the foreskin is usually fused with the glans penis at birth. The fusion gradually breaks down over a period of up to 17 years, releasing the foreskin and allowing it to become retractable.
The coverage of the glans by the foreskin in adults is a highly variable characteristic. Some men have abundant overhang when flaccid, while others do not have complete glans coverage. In a German study, Schoeberlein found that approximately 50% of uncircumcised young men had full coverage of the glans, 42% had partial coverage, and the remaining 8% were both glans and sulcus free. After adjusting for circumcision, he established that in 4% of the young men the foreskin had spontaneously atrophied (shrunk).
One of the functions of the foreskin is to protect the glans of the penis. It is normal for the foreskin to be fused with the glans in babies and not to fully retract until around puberty. Forcible retraction of the foreskin should be avoided and the child himself should be the first person to retract the foreskin. Premature retraction, which is painful, can result in infection. Regular washing under the foreskin becomes important only when the foreskin becomes detached from the glans.
Phimosis is a condition, usually considered abnormal in an adult, in which the foreskin cannot be retracted fully or at all. In humans, phimosis can be treated through the use of topical steroid ointments, stretching, preputioplasty, or circumcision. In some cases, a dorsal slit is made in the foreskin. See phimosis for more information on treatment of phimosis. Both retractile and non-retractile foreskin are normal conditions prior to puberty. 
Where the foreskin is partially retractable, a condition called paraphimosis may occur where the foreskin becomes trapped behind the glans. This is a serious condition which must be treated as a medical emergency.
Plastic surgical modification
In addition to its use as a treatment for phimosis, circumcision, among other procedures, is also practiced for religious, aesthetic, health, or hygiene reasons.
There have been many claims about the effect of circumcision on sexual function. For example the Collins, Senkul and Laumann found no adverse effect on erectile function from circumcision while Fink and Shen did.
An analysis of a national US survey by Laumann found that uncircumcised males had a higher overall rate of sexual dysfunction especially after the age of 45 years.
- “Of the 7 sexual dysfunctions considered, uncircumcised older men were more likely to experience every one of these difficulties than were their circumcised peers.”
However, Laumann also found that circumcision was positively correlated with the amount of education of the mother and was more common amongst Whites than Blacks or Hispanics, This difference in social condition is a confounding factor.
Senkul in a study from Turkey reported delayed ejaculation after circumcision as did Shenin China. Senkul said that delayed ejaculation may in fact be seen as a benefit.
- ninety-five patients were investigated on erectile function by questionnaire before and after circumcision, respectively. Eighteen patients suffered from mild erectile dysfunction before circumcision, and 28 suffered from mild or moderate erectile dysfunction after circumcision(P = 0.001). Adult circumcision appeared to have resulted in weakened erectile confidence in 33 cases(P = 0.04), difficult insertion in 41 cases(P = 0.03), prolonged intercourse in 31 cases(P = 0.04) and improved satisfaction in 34 cases(P = 0.04). Adult circumcision has certain effect on erectile function, to which more importance should be attached.
Most studies of the effect of circumcision on sexual sensitivity have focussed on the glans and ignored the foreskin itself. Circumcision opponents suggest that the foreskin may be sexually responsive, given evidence from anatomical studies.   
There is evidence that circumcision does not lead to increased keratinization  or reduction in sensitivity of the glans . However, some make unscientific claims that circumcision reduces glans sensitivity.
Fink in his study reported: "Adult circumcision appears to result in worsened erectile function (p = 0.01), decreased penile sensitivity (p = 0.08), no change in sexual activity (p = 0.22) and improved satisfaction (p = 0.04). Of the men 50% reported benefits and 38% reported harm. Overall, 62% of men were satisfied with having been circumcised."
Fink grouped decreased sensitivity with ejaculation taking longer than the men desired. Only one question relating to ejaculation time had a significant result; the question directly addressing sensitivity had no significant difference. It has been questioned whether this should be seen as evidence of decreased sensitivity or simply of delayed ejaculation.
Bleustein et al study tested the sensitivity of the glans penis, and found no difference between circumcised and uncircumcised men , confirming an earlier study by Masters and Johnson. Some men who undergo foreskin restoration claim that the procedure really does improve glans sensitivy. Some have therefore argued that much of this perceived "improved sensitivity" of the glans reported by foreskin restorers is psychosexual and psychosomatic and an example of the placebo effect, with no real change in glans sensitivity. 
Yang et al. concluded in their study into the innervated of the penile shaft and glans penis that: "The distinct pattern of innervation of the glans emphasizes the role of the glans as a sensory structure."
Circumcision opponents point out that studies on the effect of circumcision on sexual sensitivity have focussed on the glans and ignored the foreskin itself, citing    as studies which report great sensitivity and abundant nerve supply. While there are no studies examining whether it or any other single part of the penis is specifically sexually sensitive, they claim a sexual role based upon the presence of nerve-endings in the foreskin sensitive to light touch and fluttering sensations. Not all agree, however. Anti-circumcision activist and author Kristen O'Hara argues that the glans is the "male clitoris", with the foreskin maintaining at best a supporting role during sexual intercourse.
Retired pathologist and anti-circumcision activist John R. Taylor Taylor has postulated that the highly innervated and vascularised ridged band at the end of the foreskin, which is almost invariably removed in circumcision, is erogenous and plays a role in normal sexual function. He also speculates that the gliding action possible only where sufficient loose penile shaft skin exists serves to stimulate this band through contact with the corona of the glans penis during vaginal intercourse. No scientific confirmation of Taylor's speculation exists.
Williamson and Williamson made a 1988 survey of new mothers. They found that 71% preferred a circumcised partner for sexual intercourse, and 83% for giving fellatio. When asked why, 92% responded that it stays cleaner and 90% that it looks sexier. Although 78% of the women had not had direct contact with an uncircumcised penis, those who did expressed the same preference This study was done in an area of the United States where circumcision was the norm. Another survey (1999) of female anti-circumcision activists who had had sexual experience with both circumcised and intact partners reported an overwhelming preference (six out of seven) for the uncircumcised male as a sexual partner.
Preputioplasty is an alternative surgical treatment for non-retractile foreskin . Non-surgical treatments also exist for this condition, with varying degrees of success.
Religious and Cultural Views
Both Jews and Moslems, in most interpretations of religious doctrine, practice ritual circumcision whereby the foreskin of a young boy or young man is cut away. In Judaism, an expert circumcisor known as a mohel performs the ritual, known as brit milah on the boy's eighth day after birth. It is considered to be religiously mandated by the Torah as the mark of the Covenant between God and Abraham, the forefather of both the ancient Hebrews, and the sons of Ishmael. The purported foreskin of Jesus, known as the Holy Prepuce, has been venerated as a Christian relic in various places at various times, sometimes even in several places at once. Islamic tradition does not specify when a male needs to be circumcised, as long as he is before marriage.
Many infant circumcisions, particularly in the United States, are carried out for non-religious reasons. As adults, some men undergo circumcision for comfort, appearance, or a sense of completion. Other men may choose foreskin restoration for similar reasons, which provides a man with a replacement of the lost foreskin, usually through non-surgical tissue expansion techniques.
In the Filipino culture, young boys about 8 years old, are circumcised as part of a cultural rite of passage for boys into adolescence.