Pretreatment of lignocellulosic biomass is an essential step to obtain sugars from such biomasses, aimed at breaking the recalcitrant structure of lignocellulose and facilitate the access of enzymatic hydrolytic agents to carbohydrates. Among the variety of pretreatment technologies that have been investigated in the past years, extrusion is a promising thermo-mechanical pretreatment. It is a continuous process, highly versatile, having good mixing and heat transfer capabilities and being able to operate at high solids loadings. However, its energetic and economic feasibility needs still to be evaluated before it can be an actual alternative to conventional biomass pretreatments. With the aim to provide a further insight into extrusion pretreatment, this work reviews its fundamentals and the influence of the most relevant operation variables (screw profile and speed, residence time, temperature, liquid to solid ratio and downstream operations) on extrusion performance, analyzed according to the existing bibliography. To complete this overview, the effects of extrusion on lignocellulosic biomass structure, composition and enzymatic digestibility are studied, with a focus on the performance of catalyzed extrusion. (C) 2017 Elsevier Ltd. All rights reserved.