Cellulase enzymes from fungi and bacteria form the basis of most industrial enzyme cocktails for deconstructing biomass to fermentable sugars for biofuels production. Cellulases are typically multi-modular proteins with carbohydrate binding modules and catalytic domains connected by flexible, glycosylated linkers. Understanding how these enzymes act at solid-liquid interfaces to break down biomass and subsequently engineering them for higher performance is of significant importance for the development of economically-viable biofuels. To that end, our group uses various molecular simulation methods to understand both cellulases and cellulose, with the overall aim of developing a comprehensive, molecular-level picture of how these enzymes function. This talk will review several of the recent discoveries made in our group, including new functions for cellulase sub-domains, a detailed thermodynamic examination of the various crystal forms of cellulose, and identification of new strategies for improving cellulase performance.