This review comes up with the possible association between mast cells and tumour progression and summarizes some of the most recent data on the subject. The accumulation of mast cells around tumour areas is a very old observation. However, the functional significance of such phenomenon is a subject of controversy because of contradictory experimental data. In this review, two hypotheses are suggested. The first, refers on the possibility that the accumulation of mast cells is part of a general immunological host-defense reaction since, mast cells have been shown to be cytotoxic for some tumours (especially those sensitive to tumor necrosis factor-alpha). However, if such hypothesis is correct, one should explain why in most clinical and experimental cases, tumours continue to progress although the high incidence of such immune's system cells. We are therefore brought to consider a second possibility, in which, mast cells products could promote tumoural growth and metastasis. In fact, it is well documented that heparin, combined to a range of heparin-binding factors such as bFGF or TGF beta is able to promote neovascularisation, and that mast cell proteases cause cell structural alterations and loss of the extracellular matrix integrity. The role of histamine secreted by mast cells is less clear. There is indeed controversial experimental data referring to histamine's content within tumoural tissues and to histamine's proper effect on tumour expansion. Finally, this review discuss the mechanisms resulting to mast cell accumulation around tumours and more particularly the contribution of tumoural cells.