Intracellular Ca 2+ signaling drives angiogenesis and vasculogenesis by stimulating proliferation, migration, and tube formation in both vascular endothelial cells and endothelial colony forming cells (ECFCs), which represent the only endothelial precursor truly belonging to the endothelial phenotype. In addition, local Ca 2+ signals at the endoplasmic reticulum (ER)–mitochondria interface regulate endothelial cell fate by stimulating survival or apoptosis depending on the extent of the mitochondrial Ca 2+ increase. The present article aims at describing how remodeling of the endothelial Ca 2+ toolkit contributes to establish intrinsic or acquired resistance to standard anti-cancer therapies. The endothelial Ca 2+ toolkit undergoes a major alteration in tumor endothelial cells and tumor-associated ECFCs. These include changes in TRPV4 expression and increase in the expression of P2X7 receptors, Piezo2, Stim1, Orai1, TRPC1, TRPC5, Connexin 40 and dysregulation of the ER Ca 2+ handling machinery. Additionally, remodeling of the endothelial Ca 2+ toolkit could involve nicotinic acetylcholine receptors, gasotransmitters-gated channels, two-pore channels and Na +/H + exchanger. Targeting the endothelial Ca 2+ toolkit could represent an alternative adjuvant therapy to circumvent patients’ resistance to current anti-cancer treatments.