This article draws on a range of primary and secondary sources to provide an account of the development in England of sex education in schools between 1870 and 2000. It pays particular attention to the decades prior to and just after the Second World War (1939-45) about which relatively little is known. Focusing on official guidance to schools provided by central government, and reports of the nature and extent of practices in school sex education at the local level, the article traces gradual shifts in policy on and practices of sex education. It is shown that, although some formal sex education took place in schools during the early part of the twentieth century, matters of sex did not become firmly established in central government guidance to schools until the 1950s. Thereafter, both central government guidance on sex education and local level practices in schools became more expansive. However, the 1980s and 1990s stand out as countervailing decades in the longer term progressive development of school sex education in England. (Contains 1 note.)