Pretreatment is an important tool for practical cellulose conversion processes, and is the subject of this article. Pretreatment is required to alter the structure of cellulosic biomass to make cellulose more accessible to the enzymes that convert the carbohydrate polymers into fermentable sugars as represented in the schematic diagram of Fig. 1. The goal is to break the lignin seal and disrupt the crystalline structure of cellulose. Pretreatment has been viewed as one of the most expensive processing steps in cellulosic biomass-to-fermentable sugars conversion with costs as high as 30¢/gallon ethanol produced. Pretreatment also has great potential for improvement of efficiency and lowering of cost through research and development