A general review of the nature and classification of tufas is presented and the available literature is summarised. An attempt is made to standardise the terminology currently in use and to distinguish clearly between ambient temperature deposits (tufas), thermal deposits (travertines) and speleothems. Consideration is also presented of the physico-chemical and biological processes, often acting together, which are responsible for the precipitation of freshwater calcium carbonate within tufa systems. These processes appear to be climatically controlled. Therefore, tufas may be of value in palaeo-environmental reconstruction, especially if intercalated with peaty material. While the majority of tufa deposits are of post-glacial age some of the most spectacular carbonate precipitates are thermal travertines. The second part of the paper deals with a world-wide survey of the principal deposits of tufa and travertine. Space prevents a fuller account of the European deposits and the reader is preferred to Pentecost (1995) for a wide range of specific examples from Europe and Asia Minor.