Remember, glycolysis can take place regardless of whether or not we actually have oxygen present in the cell. So glycolysis is the process, the first process of aerobic respiration, and it's the first process of anaerobic respiration as well. However, under anaerobic conditions, glycolysis is the only method by which we produce ATP molecules in the cell. Now, the problem with this is glycolysis uses up NAD plus molecules. But in order for glycolysis to continually take place, we have to somehow continually regenerate the NAD plus molecules from the NADH molecules. And the way that we regenerate these NAD plus molecules that are needed by glycolysis is by the process of fermentation.