Intracellular environment is characterized by high content of various biomacromolecules, limited amount of free water, and almost complete lack of unoccupied space. Since such extremely crowded milieu is poorly modeled by slightly salted aqueous solutions of low concentrations of a biomolecule of interest, in the laboratory practice, it is typically mimicked by concentrated solutions of various polymers or inert proteins that serve as model “crowding agents”. Analysis of the behavior of different proteins under these conditions that model the effects of macromolecular crowding in vitro revealed that protein structure, folding, shape, conformational stability, binding of small molecules, enzymatic activity, protein-protein interactions, protein-nucleic acid interactions, and pathological aggregation are all affected by the presence of crowders. The goal of this chapter is to provide an overview of the variety of effects of macromolecular crowding on a protein molecule.