Studies on the preparation and catalytic action of zeolites in the form of nanosized crystals are reviewed. Examples of successful synthesis of nanosized zeolites of the FAU, MFI, BEA, and MOR structural types and their use in such processes of petroleum chemistry and refining as catalytic cracking, hydrocracking, hydroisomerization, oligomerization, transalkylation, conversion of methanol and acetone to olefins, and other processes are given. The progress and prospects of using nanosized zeolites in slurry reactors comprising three-phase systems, in which individual zeolite particles are dispersed in a liquid medium, are considered.