Mast cells (MC) are fascinating, multifunctional cells that play a prominent role in allergic inflammatory responses, mediated by activation of these cells via the allergen-immunoglobulin E (IgE) axis. Due to their ability to rapidly express mediators, cytokines, and growth factors, they have expanded roles in the genesis of autoimmunity, vascular disease, and malignancy. Because of their proximity to vasculature and their role in the secretion of pro-angiogenic molecules, mast cells have been reported to be involved with tumor angiogenesis. Mast cells participate in many aspects of tumor development, progression, and invasiveness as well as, in certain cancers, in inhibitory effects as well. Cell-cell interactions in tumor microenvironments (TME) involving mast cells, eosinophils, platelets, cancer cells, fibroblasts, T lymphocytes, and other adventitial cells create a rich milieu that could promote tumor growth. Mediators secreted by mast cells including proteases play major roles in tumor biology and invasiveness. Secretion of cytokines such as IL-33, growth factors, and tumor-promoting cytokines as well as tumor angiogenesis factors by mast cells have led to the possibility of novel therapeutics in difficult cancers. These may use concepts derived from allergy and hematology (such as inhibitors of tyrosine kinases, tryptase, cytokines such as interleukin-4 and IL-5), leading to new families of primary or adjunctive therapeutics for advanced malignancy.