Mast cells are a component of cancer microenvironment the role of which is complex and poorly understood. Mast cells promote cancer growth by stimulation of neoangiogenesis, tissue remodeling and by modulation of the host immune response. The mediators of cancer promotion include protease-activated receptors, mitogen activated protein kinases, prostaglandins and histamine. Histamine may induce tumor proliferation and immunosuppression through H1 and H2 receptors, respectively. The mast cell-derived modulators of immune response include also interleukin 10 (IL-10), tumor necrosis factor α (TNF-α) and CD30L. Possibly stimulation of angiogenesis is the most important. Mast cells release potent proangiogenic factors such as vascular endothelial growth factor (VEGF), basic fibroblast growth factor (bFGF), transforming growth factor β (TGF-β), TNF- α and IL-8, and mast cells' enzymes, like metaloproteinases (MMPs), tryptase and chymase participate in vessels' formation. The anti-cancer actions of mast cells include direct growth inhibition, immunologic stimulation, inhibition of apoptosis and decreased cell mobility; the mediators of these processes include chymase, tryptase, TNF-α, IL-1 and IL-6. The very same mediators may exert both pro- or anti-cancer effects depending on concentration, presence of cofactors or location of secreting cells. In fact, peri- and intra-tumoral mast cells may have dissimilar effects. Understanding of the role of mast cells in cancer could lead to improved prognostication and development of therapeutic methods targeting the mast cells.