Reliance on glutamine has long been considered to be a hallmark of cancer cell metabolism. However, some recent studies have challenged this notion in vivo, prompting a need for further clarification of the role of glutamine metabolism in cancer. We find that there is ample evidence of an essential role for glutamine in tumors, and that a variety of factors, including tissue type, the underlying cancer genetics, the tumor microenvironment, and other variables such as diet and host physiology collectively influence the role of glutamine in cancer. Thus the requirements for glutamine in cancer are overall highly heterogeneous. In this review we discuss the implications both for basic science and for targeting glutamine metabolism in cancer therapy.