The fact that tumors are composed of both tumor cells and host cells has long been known. These tumor-associated cells include vascular endothelial cells and pericytes, as well as inflammatory cells such as neutrophils, monocytes, macrophages, mast cells, eosinophils, and lymphocytes. The tumor cells also interact with stromal cells and with elements of the tissue extracellular matrix. What has been less appreciated is the role that these cells could have in modulating the growth, invasion, and metastasis of the tumor. Early on, the elements of what we now call the tumor microenvironment were considered to be more or less innocent bystanders to the role of the tumor cells as they grew and invaded local sites. Today, there is an increased understanding of the critical role of the tumor microenvironment in dramatically influencing the course of tumor development and dissemination.