Second-generation ethanol production from various lignocellulosic materials based on enzymatic hydrolysis of cellulose has moved from research in lab scale to pilot- and demo scale but has not yet reached commercial scale. One of the crucial process steps is the pretreatment of the biomass, which has as primary aim to make the biomass accessible to enzymatic attack, as it has a large impact on all the other steps in the process. Several pretreatment methods have been developed, comprising methods working at low pH, i.e., acid based, at medium pH (without addition of catalysts), or at high pH, i.e., with a base as catalyst. Many methods result in high sugar yields, above 90% of theoretical for agricultural residues while more recalcitrant materials like hardwood, and especially softwood, require dilute-acid pretreatment to reach high sugar yields. However, most studies on pretreatment have been assessed by enzymatic hydrolysis at low solids content and high enzyme dosages. The various pretreatment methods need in the future to be reassessed at more industrial-like conditions considering the whole integrated process taking into consideration the influence on all process steps. In this review, various pretreatment methods are discussed and how assessment should be performed to reach optimal conditions. (C) 2012 Elsevier Ltd. All rights reserved.