Enzymes have been incorporated in nanostructures in order to provide robust catalysts for valuable reactions, particularly those performed under harsh and denaturing conditions. This chapter describes the encapsulation of enzymes in polyacrylamide nanogels by a two-step in situ polymerization process for preparing robust biocatalysts. The first step in this process is the generation of vinyl groups on the enzyme surface, while the second step involves in situ polymerization using acrylamide as the monomer. Encapsulation of the enzyme in the hydrophilic, porous, and flexible polyacrylamide gel of several nanometers thick would help to both give a significantly enhanced thermostability and prevent the removal of essential water by polar solvents. The hydrophilic flexible polymer shell also allows the protein structure to undergo necessary conformational transitions during the catalytic reaction and, at the same time, impose marginal mass transfer restrictions for the substrates entering across the polymer shell. The effectiveness of this method is demonstrated with horseradish peroxidase (HRP), carbonic anhydrase, and lipase. The impacts of such an encapsulation on the activity and stability of enzymes are also discussed.