Intracellular space is highly crowded with soluble and insoluble biomolecules that range from large polymers, such as proteins and nucleic acids, to small molecules, including metabolites and metal ions. It is therefore of interest to understand the effects of molecular crowding on the structure, stability, and function of biomolecules. Moreover, molecular crowding is observed not only intracellularly but also in the extracellular matrix and under the conditions used in in vitro biotechnological and nanotechnological processes. However, most biochemical studies of biomolecules are performed under dilute conditions. Recent studies have demonstrated critical effects of molecular crowding on nucleic acids. In the present study we discuss how molecular crowding affects the properties of G-quadruplexes as well as other non-canonical nucleic acid structures.