Alkaline pretreatment significantly increases the digestibility of lignocellulose. Pretreatment conditions (e.g., time, temperature, pressure, alkali concentration) vary widely depending on the selected feedstock and alkali. This chapter reviews the basic chemistry that occurs during alkaline pretreatment, which involves the main lignocellulose polymers (lignin, cellulose, and hemicellulose). In addition, it describes important results obtained from widely applied alkaline pretreatments.