For immune surveillance to be successful in preventing cancer development, the immune system has to function robustly. For immunotherapy vaccines, effector cells, and antibodies to be successful at eradication of existing tumors, the immune system must function well. This chapter describes the many ways in which the presence of cancer can impact and deregulate immune function in patients. It presents current understanding of immune function at the tumor site, and immune function throughout the human system as generally measured in the blood. The chapter also focuses on specific cell types and molecules that have important protumor and antitumor effects. It begins with an examination of tumor infiltrates and focuses on the function and characterization of myeloid cells and their immune suppressive capabilities. The chapter presents critical molecular mechanisms and cellular networks of antitumor activity and discusses how complex immune activity is affected by the basic biology of metabolism and aging.