Despite many years of intensive effort, there is surprisingly little consensus on the most suitable markers with which to locate and isolate stem cells from adult tissues. By comparison, the study of cancer stem cells is still in its infancy; so, unsurprisingly, there is great uncertainty as to the identity of these cells. Stem cell markers can be broadly categorized into molecular determinants of self-renewal, clonogenicity, multipotentiality, adherence to the niche, and longevity. This review assesses the utility of recognizing cancer stem cells by virtue of high expression of aldehyde dehydrogenases (ALDHs), probably significant determinants of cell survival through their ability to detoxify many potentially cytotoxic molecules, and contributing to drug resistance. Antibodies are available against the ALDH enzyme family, but the vast majority of studies have used cell sorting techniques to enrich for cells expressing these enzymes. Live cells expressing high ALDH activity are usually identified by the ALDEFLUOR kit and sorted by fluorescence activated cell sorting (FACS). For many human tumours, but notably breast cancer, cell selection based upon ALDH activity appears to be a useful marker for enriching for cells with tumour-initiating activity (presumed cancer stem cells) in immunodeficient mice, and indeed the frequency of so-called ALDHbri cells in many tumours can be an independent prognostic indicator. Copyright © 2010 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.