A close interaction of cancer cells with their microenvironment is important for their growth and survival. In this respect, the involvement of inflammatory cells in the initiation, promotion and progression of cancer has pointed to new therapeutic opportunities in the treatment of cancer. The main immune cell types implicated in tumor-associated inflammation are macrophages, dendritic cells, lymphocytes, neutrophils, eosinophils and mast cells. Their precise role in intercellular communication, regulation of tumor inflammation, and to what respect this inflammation contributes to tumor development, are not completely understood. Mast cells are key effector cells in allergic diseases, but it has become apparent that they also contribute to other pathologies, including autoimmune diseases and cancer. Activated mast cells can release many pro-angiogenic and tumor growth stimulatory mediators. Increased numbers of mast cells are found in many tumors and it has been shown that the number of tumor infiltrating mast cells correlate with increased intratumoral microvessel density, enhanced tumor growth and tumor invasion, and poor clinical outcome. Therefore, modulating mast cell recruitment, viability, activity, or mediator release patterns at malignant sites can be of importance to control tumor growth. In this review, we will focus on the contribution of mast cells to tumor development and growth and the possibilities to interfere in mast cell activation and proliferation in the therapy of cancer.