Loess can be defined simply as a terrestrial clastic sediment, composed predominantly of silt-size particles, which is formed essentially by the accumulation of wind-blown dust. There are three fundamental requirements for its formation: (1) a sustained source of dust, (2) adequate wind energy to transport the dust, and (3) a suitable accumulation site. Most loess has been modified to some degree by local reworking, bioturbation and syn-depositional weathering, but a process of 'loessification' is not necessary for a dust deposit to qualify as loess. The terms 'primary loess' and 'secondary loess' have previously been used to described wind-deposited loess and redeposited loess, respectively, but it is more accurate to describe material reworked and redeposited by non-aeolian processes as 'loess-derived colluvium' or 'loess-derived alluvium'. During the Quaternary, loess formed in three main situations: (1) mid-continental shield areas beyond the limits of ice sheets (periglacial loess), (2) on the margins of high mountain ranges (perimontane loess), and (3) on the semi-arid margins of some lowland deserts (peridesert loess). Peridesert loess is relatively poorly developed partly due to the limited efficiency of silt-generating mechanisms in low-land deserts compared with glaciation, but also important is the tendency in arid areas for dust to be dispersed over a wide area and to be rapidly reworked after deposition, particularly in areas which experience frequent climatic changes. The remarkably thick loess of China and Central Asia results from an unusual combination of conditions which has persisted for much of the last 2-3 million years, namely rapid uplift of the Tibetan Plateau and surrounding mountain ranges, high rates of sediment production and supply to adjacent basins, a strong northwesterly and westerly wind regime, and the existence of effective dust traps downwind of the source regions.