In plant physiology, the Warburg effect is the decrease of photosynthesis by high oxygen concentrations. Oxygen is a competitive inhibitor of the carbon dioxidefixation by RuBisCO which initiates photosynthesis. Furthermore oxygen stimulates photorespiration which reduces photosynthetic output. These two mechanisms working together are responsible for the Warburg effect.
In oncology, the Warburg effect is the observation that most cancer cells predominantly produce energy by a high rate of glycolysis followed by lactic acid fermentation in the cytosol, rather than by a comparatively low rate of glycolysis followed by oxidation of pyruvate in mitochondria as in most normal cells. The latter process is aerobic (uses oxygen). Malignant, rapidly growing tumor cells typically have glycolytic rates up to 200 times higher than those of their normal tissues of origin; this occurs even if oxygen is plentiful.
The Warburg effect may simply be a consequence of damage to the mitochondria in cancer, or an adaptation to low-oxygen environments within tumors, or a result of cancer genes shutting down the mitochondria because they are involved in the cell's apoptosis program which would otherwise kill cancerous cells. It may also be an effect associated with cell proliferation. Since glycolysis provides most of the building blocks required for cell proliferation, cancer cells (and normal proliferating cells) have been proposed to need to activate glycolysis, despite the presence of oxygen, to proliferate . Evidence attributes some of the high aerobic glycolytic rates to an overexpressed form of mitochondrially-bound hexokinase responsible for driving the high glycolytic activity. In kidney cancer, this effect could be due to the presence of mutations in the Von Hippel–Lindau tumor suppressor gene upregulating glycolytic enzymes, including the M2 splice isoform of pyruvate kinase 
In March 2008, Lewis C. Cantley and colleagues at the Harvard Medical School announced they had identified the enzyme that gave rise to the Warburg effect. The researchers stated tumor M2-PK, a form of the pyruvate kinase enzyme, is produced in all rapidly dividing cells, and is responsible for enabling cancer cells to consume glucose at an accelerated rate; on forcing the cells to switch to pyruvate kinase's alternative form by inhibiting the production of tumor M2-PK, their growth was curbed. The researchers acknowledged the fact that the exact chemistry of glucose metabolism was likely to vary across different forms of cancer; but PKM2 was identified in all of the cancer cells they had tested. This enzyme form is not usually found in healthy tissue, though it is apparently necessary when cells need to multiply quickly, e.g. in healing wounds or hematopoiesis.
Many substances have been developed which inhibit glycolysis, and such inhibitors are currently the subject of intense research as anticancer agents, including SB-204990, 2-deoxy-D-glucose (2DG), 3-bromopyruvate (3-BrPA, bromopyruvic acid, or bromopyruvate), 3-BrOP, 5-thioglucose and dichloroacetic acid (DCA). Clinical trials are ongoing for 2-DG and DCA.
Alpha-cyano-4-hydroxycinnamic acid (ACCA;CHC), a small-molecule inhibitor of monocarboxylate transporters (MCTs; which prevent lactic acid build up in tumors) has been successfully used as a metabolic target in brain tumor pre-clinical research. Higher affinity MCT inhibitors have been developed and are currently undergoing clinical trials by Astra-Zeneca.
A model called the reverse Warburg effect describes cells producing energy by glycolysis, but were not tumor cells, but stromal fibroblasts. Although the Warburg effect would exist in certain cancer types potentially, it highlighted the need for a closer look at tumor metabolism. 
Cancer metabolism and epigenetics
Nutrient utilization is dramatically altered when cells receive signals to proliferate. Characteristic metabolic changes enable cells to meet the large biosynthetic demands associated with cell growth and division. Changes in rate-limiting glycolytic enzymes redirect metabolism to support growth and proliferation. Metabolic reprogramming in cancer is controlled largely by oncogenic activation of signal transduction pathways and transcription factors. Although less well understood, epigenetic mechanisms may seem to contribute to the regulation of metabolic gene expression in cancer. Reciprocally, accumulating evidence suggest that metabolic alterations may affect the epigenome. Understanding the relation between metabolism and epigenetics in cancer cells may open new avenues for anti-cancer strategies.
^Lopez-Lazaro M (2008). "The Warburg effect: why and how do cancer cells activate glycolysis in the presence of oxygen?". Anticancer Agents Med. Chem.8 (3): 305–312. doi:10.2174/187152008783961932. PMID18393789.
^Unwin, Richard D.; Craven, Rachel A.; Harnden, Patricia; Hanrahan, Sarah; Totty, Nick; Knowles, Margaret; Eardley, Ian; Selby, Peter J.; Banks, Rosamonde E. (1 August 2003). "Proteomic changes in renal cancer and co-ordinate demonstration of both the glycolytic and mitochondrial aspects of the Warburg effect". PROTEOMICS3 (8): 1620–1632. doi:10.1002/pmic.200300464.
^Pedersen PL (2007). "Warburg, me and Hexokinase 2: Multiple discoveries of key molecular events underlying one of cancers' most common phenotypes, the "Warburg Effect", i.e., elevated glycolysis in the presence of oxygen". J. Bioenerg. Biomembr.39 (3): 211–222. doi:10.1007/s10863-007-9094-x. PMID17879147.
^Bonnet S, Archer SL, Allalunis-Turner J, Haromy A, Beaulieu C, Thompson R, Lee CT, Lopaschuk GD, Puttagunta L, Bonnet S, Harry G, Hashimoto K, Porter CJ, Andrade MA, Thebaud B, Michelakis ED (2007). "A mitochondria-K+ channel axis is suppressed in cancer and its normalization promotes apoptosis and inhibits cancer growth". Cancer Cell11 (1): 37–51. doi:10.1016/j.ccr.2006.10.020. PMID17222789.