HP is a cubic-grain black-and-white film from Ilford Photo with a long history. It originated as Hypersensitive Panchromaticplates in 1931. Since then it progressed through a number of versions, with HP5 plus (HP5+ for short) being the latest. The main competitor of Ilford HP5 Plus is Kodak Tri-X 400.
HP3 went through two speed changes in its history, but only one actual change to the emulsion. In 1960 the 200 ASA emulsion was relabelled to 400 ASA with no change to the product. The 200 ASA speed included an exposure safety margin, but with improvements in light meters this was deemed unnecessary. Thus the speed was revised up to 400 ASA.
On September 23, 2005 Ilford reintroduced their black-and-white single-use camera which includes 27 exposures of HP5 plus film.