Having been described in 1904 by Dubois as a locality for fossil mammals, the Tegelen clay-pits are nowadays considered a ‘classical’ locality. It is the type locality of the Tiglian, a warm period of the Early Pleistocene or Late Pliocene. The pits are primarily known for their mammalian remains, but have also yielded seeds, pollen and freshwater snails. A century of collecting has resulted in extensive collections of large mammals in various museums, the most important of which are the Teylers Museum, Haarlem and the National Museum of Natural History Naturalis in Leiden. The latter museum also holds a large collection of microvertebrates, collected during campaigns in the 1970s. These campaigns showed that, in spite of the numerous fossils, the Tegelen Clay is in fact a relatively fossil-poor locality. Collections were assembled by workers, processing vast amounts of clay for the local ceramic industry. Thus, although technically an in situ locality, the exact provenance of the various fossils is largely uncertain.