Fossiliferous Karst Deposits of Atapuerca. First 20 years of research. The conception of a multidisciplinary research project on the ancient occupants of Spain and Europe, and the ecological contexts of their evolution derived in 1976 from the incidental find of 16 human fossils in a deep pit, SH, well into the ‘Cueva Mayor’ of Sierra de Atapuerca (Ibeas de Juarros, Burgos, Spain), and after visiting several outcrops of deep, well stratified karst deposits in an abandoned railway cut, not far. The human fossils from SH were assigned to the European group of preneandertalians (Aguirre 1976; Aguirre & de Lumley, 1977), and classified as Homo sapiens heidelbergensis (Aguirre & Rosas, 1985). A ‘mosaic’ evolution was then recognized on the morphological traits of human mandibles in Middle Pleistocene, also an African origin of the pre-neandertalians, and the ancestorship of the latter relate to neandertalians. Different approaches led by 1987 to estimate the age of SH hominines c. 300/320 Ka. BP. Paleopathological and taphonomical studies allowed inferring good health overall; cultural factors were suggested for several common deficiencies. In 1990 the SH site was finally cleaned and human bones in situ exposed after removing more than 6 Tm of a mass of accumulated stones, mud and bone fragments removed by searchers of bear teeth. All that overburden washed and shifted yielded thousands of bear fossils and more then 250 human pieces of a MNI = 11. Excavation of untouched deposit between 1990 and 1999 produced more than 2 400 human fossils belonging to a MNI = 27. Studies of paleodemography are published, based on the assumption that the corpses were deposited as death occurred along time and as a behavioural pattern; present author thinks that the simultaneous death of a group taking refuge in a cave under heavy rain, and land-slide following, is more consistent with observed evidences. Two sites on the railway cut were sampled: the Gran Dolina (TD), 18.5 m deep, and Galeria (TG) with a connected vertical shaft (TN) in the ‘Complejo Tres Simas’, yielding abundant microvertebrate fossils and continuous or dicontinuous pollen record. Extension excavation started in both TD and TG sites over c. 24 m 2 in each met successive horizons with mammal assemblages known in European sites of the upper third of Middle Pleistocene and evidences of human occupation. Basal beds of TD also were excavated in a talus: these yielded a paleofauna known as Upper Bhiharian, or Early Cromerian. The extreme ages obtained for TD are c. 900 and c. 150 Ka. Lithic assemblages, analysed from the operational viewpoint, belong to Mode 1 in TD6, to a particularly evolved Mode 2 in TG, to early Mode 3 in upper TD beds. Microwear traces were studied on rodent and amphibian fossils to identify consumers, and on tools in search of traces of use. Differences were found in meat consumption and on use and permanence in various cavities. A slit open cut in TD started in 1992 yielded fossil humans with abundant lithic and faunal context in upper Bed TD6 well below the B/M magnetic reversal. Presently debated interpretations are indicated, and future potentials suggested.