Molecular sieves (zeolites, mesoporous materials, hierarchic systems, two-dimensional zeolites) are important acid catalysts for transformations of biomass into required products. This article covers the upgrading potential of different types of biomass based on their source and discusses the reaction pathways of their transformations emphasizing the role of molecular sieve catalysts. As typical examples, catalytic pyrolysis, deoxygenation, transesterification, dehydration, isomerization, and condensation reactions were chosen. Properties and chemical/textural characteristics of the most important molecular sieve catalysts are briefly discussed and related to their catalytic behavior in biomass transformations. Finally, potential of novel molecular sieve catalysts towards biomass upgrading is highlighted and some examples of novel zeolitic materials are addressed.