The removal of sugars and production of alcohol by yeast during fermentation. The wort (solution of extract made from grain) is thinned out - or attenuated - during this process.

Attenuation is often measured by taking the difference between the starting gravity of the unfermented wort and the final gravity of the beer. This measurement is termed "apparent attenuation" because it overestimates the extent of sugar removal from the wort.