The S100 family of calcium binding proteins has been shown to be involved in a variety of physiological function, such as cell proliferation, extracellular signal transduction, intercellular adhesion, motility as well as cancer metastasis. The role played by a member of the S100 gene family, viz. S100A4 (also referred to as mtsl, 18A2/mtsl, pEL-98, p9Ka, metastasin) in the control of cell proliferation as well as in cancer invasion and metastasis has now been extensively studied in a number of laboratories. The protein encoded by S100A4 gene is now known to be capable of regulating cell cycle progression, modulating intercellular adhesion and invasive and metastatic properties of cancer cells. The S100A4 protein appears to be able to sequester and disable the p53 suppressor protein which controls G1-S transition of cells as well as the exit of cells from the S phase into mitosis G2-M transition is believed to involve the induction of stathmin (Op18) gene expression. The expression of this gene has been found to parallel that of S100A4, S100A4 also appears to take part in the homeostasis of growth, with apparent involvement also in growth factor signal transduction and apoptotic cell death. There is considerable evidence that S100A4 expression alters the adhesive properties of cells, possibly by remodelling the extracellular matrix and promoting a redeployment of adhesion-mediating macromolecules occurring in the extracellular matrix. Using transfection technology, it has been shown that over-expression of S100A4 enhances lung colonisation by cancer cells. The transfection and expression of antisense constructs, in contrast, inhibit metastatic localisation in the lung. S100 proteins levels in serum and in tumour tissue are increasingly being monitored and have been regarded as good indicators of the state of cancer progression. Valuable evidence has accumulated regarding the expression of S100A4 in human melanomas. In carcinoma of the breast, the level of expression of S100A4 has been found to be closely related to metastatic spread of the cancer to regional lymph nodes. The purpose of this review is to emphasise the need to focus sharply upon the mechanisms by which S100 proteins in general and S100A4 in particular subserve the wide variety of functions currently attributable to them.