A Girl's Guide to 21st Century Sex is a documentary TV series about sex, which ran in eight episodes on Channel 5 and was presented by Dr. Catherine Hood. The 45-minute-long episodes (including advertisements) were broadcast on Monday nights. The series started on 30 October 2006, with the final programme broadcast on 18 December 2006.

Each episode explained a sex position and covered a sexually transmitted disease. Additionally, the following topics were discussed: sex among people with disabilities and overweight people, penis enlargement devices, penis enlargement surgery, sexual violence against men and penis removal, tantric sex, the g-spot, erectile dysfunction, sex reassignment surgery, cosmetic surgery of the vagina (labiaplasty), swinging, lichen sclerosus, the use of recreational drugs during sex, male homosexual sex in public toilets, full body plastic wrap bondage, and sex dolls.

The programme included close shots of the male and female body as well as footage of sexual intercourse and ejaculation filmed with an internal camera placed inside the vagina. These scenes were filmed starring English-born pornographic actor Stefan Hard and Australian-born pornographic actress Elizabeth Lawrence.

A second series was not commissioned in 2007 or subsequent years (with repeats until 2014).