Solid tumors are not simply clones of malignant cells. Instead, they can be considered dysfunctional organs, end-product of the altered interplay, within the tumor microenvironment (TME), among cancer cells and stromal cells (e.g. endothelial cells, macrophages, neutrophils, T cells, etc.). This concept has thrown a spotlight on the TME as the central unit governing tumor progression, metastasis and resistance to antitumor therapies.