The resurgence of research in cancer metabolism has broadened the focus beyond glucose and the Warburg Effect to include other nutrients, such as glutamine. Glutamine plays a crucial role in cell growth and cancer cell biology, and its metabolism can be exploited for therapeutic purposes. This review provides an updated overview of glutamine metabolism and its involvement in tumorigenesis, both in vitro and in vivo, and explores the potential clinical applications of basic science discoveries. Glutamine is conditionally essential for growing cells and is rapidly consumed by cancer cells for energy generation and biomass accumulation. Glutamine is transported into cells through transporters like SLC1A5 and can be used for biosynthesis or exported back out through antiporters. Cancer cells can also acquire glutamine through macromolecule breakdown under nutrient-deprived conditions. Glutamine is converted to glutamate, which is further catabolized through two pathways: the TCA cycle and transamination. The expression of enzymes involved in glutamine metabolism varies widely in cancers and is influenced by tissue of origin and oncogenotypes. Glutamine supports amino acid pools, reduces carboxylation, and fuels fatty acid synthesis through reductive carboxylation. It also suppresses the integrated stress response (ISR) and endoplasmic reticulum (ER) stress pathways, maintaining translation, protein trafficking, and survival. Glutamine controls reactive oxygen species (ROS) levels through glutathione synthesis and NADPH production. The mTOR pathway, which promotes biosynthetic pathways, is regulated by glutamine availability. Oncogenic insults and mutations, such as MYC and KRAS, upregulate glutamine metabolism. MYC-driven cells become dependent on exogenous glutamine, while KRAS-driven transformation induces dependence on glutamine metabolism. Hypoxia-induced stabilization of HIFα directs glutamine towards biosynthetic fates that do not require oxygen. In the clinic, reprogrammed cancer metabolism can be used for imaging tumors, and targeting glutamine metabolism has shown promise in preclinical models. However, understanding resistance mechanisms and developing combination therapies that induce synthetic lethality are ongoing challenges. The heterogeneity of tumors and the tumor microenvironment further complicate the use of metabolic therapies. Overall, the field of cancer metabolism has made significant progress in understanding alternative fuel sources for cancers, including glutamine, which can be exploited for therapeutic purposes.The resurgence of research in cancer metabolism has broadened the focus beyond glucose and the Warburg Effect to include other nutrients, such as glutamine. Glutamine plays a crucial role in cell growth and cancer cell biology, and its metabolism can be exploited for therapeutic purposes. This review provides an updated overview of glutamine metabolism and its involvement in tumorigenesis, both in vitro and in vivo, and explores the potential clinical applications of basic science discoveries. Glutamine is conditionally essential for growing cells and is rapidly consumed by cancer cells for energy generation and biomass accumulation. Glutamine is transported into cells through transporters like SLC1A5 and can be used for biosynthesis or exported back out through antiporters. Cancer cells can also acquire glutamine through macromolecule breakdown under nutrient-deprived conditions. Glutamine is converted to glutamate, which is further catabolized through two pathways: the TCA cycle and transamination. The expression of enzymes involved in glutamine metabolism varies widely in cancers and is influenced by tissue of origin and oncogenotypes. Glutamine supports amino acid pools, reduces carboxylation, and fuels fatty acid synthesis through reductive carboxylation. It also suppresses the integrated stress response (ISR) and endoplasmic reticulum (ER) stress pathways, maintaining translation, protein trafficking, and survival. Glutamine controls reactive oxygen species (ROS) levels through glutathione synthesis and NADPH production. The mTOR pathway, which promotes biosynthetic pathways, is regulated by glutamine availability. Oncogenic insults and mutations, such as MYC and KRAS, upregulate glutamine metabolism. MYC-driven cells become dependent on exogenous glutamine, while KRAS-driven transformation induces dependence on glutamine metabolism. Hypoxia-induced stabilization of HIFα directs glutamine towards biosynthetic fates that do not require oxygen. In the clinic, reprogrammed cancer metabolism can be used for imaging tumors, and targeting glutamine metabolism has shown promise in preclinical models. However, understanding resistance mechanisms and developing combination therapies that induce synthetic lethality are ongoing challenges. The heterogeneity of tumors and the tumor microenvironment further complicate the use of metabolic therapies. Overall, the field of cancer metabolism has made significant progress in understanding alternative fuel sources for cancers, including glutamine, which can be exploited for therapeutic purposes.