Current immune therapies for cancer have been disappointing. The various approaches to immunotherapy for cancer so far tried clinically include adoptive immunotherapy, vaccination strategies, and administration of anti-tolerogenic antibodies. Each of these approaches basically involves the inhibition or circumvention of immune tolerance, activating immune effectors that have the capability to recognize and lyse tumor. Unfortunately, only a relatively small population of patients respond to these therapies, and most of the responses are not durable. There is mounting evidence that immune interventions employing anti-tolerogenic strategies are insufficient to control tumor, as the tumor microenvironment is generally immunosuppressive. The present review summarizes the current knowledge on the cellular constituents of tumor (excluding tumor cells themselves) that contribute to this immunosuppressive microenvironment.