The tumor microenvironment plays an essential role in various stages of cancer development. This environment, composed of the extracellular matrix, fibroblasts, endothelial cells, and cells of the immune system regulates the behavior of and co-evolve with tumor cells. Many of the components, including the innate and adaptive immune cells, play multifaceted roles during cancer progression and can promote or inhibit tumor development, depending on local and systemic conditions. Interestingly, a strategy by which tumor cells gain drug resistance is by modifying the tumor microenvironment. Together, understanding the mechanisms by which the tumor microenvironment functions should greatly facilitate the development of new therapeutic interventions by targeting the tumor niche.