A tumor is an ever-evolving and growing ecosystem of cells, an unhinged organ that takes root and spreads wherever it can. Though researchers have historically focused heavily on its constituent cancer cells, it has become abundantly clear in recent years that almost all tumors also depend on a supporting cast of noncancerous cells for their survival, metastasis and resistance to therapy. These include cells that ensure blood supply for nourishment and plumbing to ferry away waste, and those that make the fibrous infrastructure of tissues. They also include a variety of immune cells that help suppress tumor-targeting immune responses and aid the unbridled growth, drug resistance and migration of cancer cells. Several Ludwig researchers are deeply involved in analyzing how these various cells support malignant growth, with the aim of targeting such dependencies for cancer therapy.