© 2016 Elsevier Inc. All rights reserved. Computer simulations based on all-atom models can provide significant assistance to researchers in understanding molecular mechanisms that lead to the appearance of new properties in composites different than those of the individual components. Recently, this simulation of polymer nanocomposites has become more popular, since it provides answers that cannot be obtained experimentally. To date, there have been only a few publications devoted to the computer simulations of polymer nanocomposites based on cellulose, even though these materials are very promising and attract great attention because of their renewable character. One of the main biodegradable polymers, obtained from natural sources and used for developing such biocomposites, is polylactide (PLA). This chapter focuses on computer simulations of PLA and nanocellulose, as well as on composite materials based on these materials.