Sex education, also known as sexual education, sexuality education or sex ed, is the instruction of issues relating to human sexuality, including human sexual anatomy, sexual activity, sexual reproduction, safe sex, birth control, sexual health, reproductive health, emotional relations, emotional responsibilities, age of consent, and reproductive rights.

Sex education that includes all of these issues is known as comprehensive sexuality education. In contrast, abstinence-only sex education, which focuses solely on promoting sexual abstinence, is often favored in more socially conservative regions, including some parts of the United States.

Sex education may be provided as part of school programs, public health campaigns, or by parents or caregivers. In some countries it is known as "relationships and sexual health education".^{[citation needed]}

Many governments see it as beneficial to provide public education on such matters prior to or at the beginning of puberty to improve public health, to limit the spread of sexually transmitted infections, and to avoid teenage pregnancy and unwanted pregnancies later on.

In multiple cultures, the discussion of all sexual issues has traditionally been considered taboo, and adolescents were not given any information on sexual matters. Such instruction, as was given, was traditionally left to a child's parents, and often this was put off until just before their marriage. However, in the late 19th century, the progressive education movement led to the introduction of sex education as "social hygiene" in North American school curricula and the introduction of school-based sex education.

During the Second World War, UK governmental concerns grew around mass relocation, parentless youths, and young men and women working together for the first time. Not only were there fears of new sexually transmitted diseases, but there was also growing anxiety around young pregnancy putting pressure on the war-ravaged economy and healthcare system. As such, the UK Board for Education introduced the Sex Education in Schools and Youth Organizations guidance. This put the onus of sex education on schools and youth groups, and guided leaders on how to execute this. For example, the mechanics of sexual intercourse could be communicated via "the keeping of livestock", as students could observe reproduction in real-time; the guidance also encouraged discussions about menstruation, motherhood courses, and personal hygiene talks. Popular among teachers and some parents, this guidance – which made sex education a possibility, not an obligation – prevailed for a number of years in the UK. In the 1970s, informational films became popular among teachers. Martin Cole's Growing Up (1971) was a frank look at how sex works physiologically and socially. It showed real clips of penises and masturbation, which sparked some backlash. However, it became apparent in the 1980s that a frank and factual approach was required in sex education as the HIV/AIDS crisis began in the UK. In 1999, the Labour government introduced Sex and Relationships Education guidance, with particular focus on sexually transmitted diseases and teenage pregnancy. This was part of the ten-year Teenage Pregnancy Strategy, which would eventually resolve in 2010. Teenage pregnancy rates were halved across this period, however similar changes in other countries indicate that this was not an effect of the strategy.

After the Second World War, some developing countries promoted sex education programs that evolved to address political goals. A growing anxiety in some areas of the world over rising birth rates led to population-centered sex education programs. For instance, the first sex education curriculum in Singapore between 1966 and 1973 emphasized birth control as a way to avoid overpopulation. Reforms in some socialist countries focused on the role of sex education in strengthening family ties within society. This was the focus of sex education programs that developed in the German Democratic Republic (GDR) and Cuba during the late twentieth century. The evolving content of sex education programs reflected shifting opinions regarding sexuality within each society. For example, Swedish sex education guidelines and textbooks published between 1945 and 2000 originally depicted masturbation as inherently harmful but increasingly portrayed it as natural and harmless.

Globally, the outbreak of AIDS has given a new sense of urgency to sex education. In multiple African countries where AIDS is at epidemic levels (see HIV/AIDS in Africa) sex education is seen by most scientists as a vital public health strategy. Some international organizations such as Planned Parenthood consider that broad sex education programs have global benefits, such as controlling the risk of overpopulation and advancing women's rights, including reproductive rights. The use of mass media campaigns has sometimes resulted in high levels of awareness coupled with essentially superficial knowledge of HIV transmission.