Before 1940, when the term "fluidization" was coined to describe a gas-solids contacting Process, liquid-fluidized beds of granular solids had long been used for classification of minerals by size (sizing) or density (sorting), and were referred to in the ore dressing literature as "teeter columns" or as being in the "teeter condition". Liquid fluidization or "teetering" has subsequently achieved several other applications, in recent years most notably as extended electrodes for electrowinning metals from dilute solutions, as self-cleaning heat exchangers and as bio-catalytic reactors. Since several important applications, in addition to classification per se, involve particle stratification by size and/or density, the main focus of this paper is on the current understanding of particle segregation and mixing in liquid-fluidized beds. (c) 2004 Elsevier B.V. All rights reserved.