Metabolism is the fundamental process for all cellular functions. For decades, there has been growing evidence with regard to the relationship between metabolism and malignant cell proliferation. Unlike normal differentiated cells, however, cancer cells have reprogrammed metabolisms in order to fulfill their energy requirements. These cells display crucial modifications in many metabolic pathways, including glucose transport, glutaminolysis which includes the tricarboxylic acid (TCA) cycle, the electron transport chain (ETC), and the pentose phosphate pathway (PPP)[1]. Since the discovery of the Warburg effect, it has been shown that the metabolism of cancer cells plays a critical role in cancer survival and growth. More recent research suggests that the involvement of glutamine in cancer metabolism is more significant than previously thought. Glutamine, a non essential amino acid with an amine functional group, is the most abundant amino acid circulating in the bloodstream [2]. This chapter will discuss the characteristic features of glutamine metabolism in cancers.