Glycolysis is the metabolic process that converts glucose into pyruvate, producing ATP and NADH in the process. It is the first stage in cellular respiration and occurs in the cytoplasm of all living cells. Glycolysis involves a series of enzymatic reactions that break down glucose into two molecules of pyruvate. The energy released during glycolysis is used to produce ATP, which is the primary source of energy for cellular processes. Glycolysis does not require oxygen, making it an anaerobic process, but it is a precursor to aerobic respiration, which does require oxygen.