Living cells contain a variety of biomolecules including nucleic acids, proteins, polysaccharides, and metabolites as well as other soluble and insoluble components. These biomolecules occupy a significant fraction (20-40%) of the cellular volume. The total concentration of biomolecules reaches 400gL(-1), leading to a crowded intracellular environment referred to as molecular crowding. Therefore, an understanding of the effects of molecular crowding conditions on biomolecules is important to broad research fields such as biochemical, medical, and pharmaceutical sciences. In this review, we describe molecular conditions in the cytoplasm and nucleus, which are totally different from in vitro conditions, and then show the biochemical and biophysical consequences of molecular crowding. Finally, we discuss the effect of molecular crowding on the structure, stability, and function of nucleic acids and the significance of molecular crowding in biotechnology and nanotechnology.