This chapter comprehensively reviews the effects of molecular crowding on the structurе, folding, and thermodynamic stability of various DNA structures. While traditional studies often utilize dilute solutions, cellular environments are characterized by high macromolecular concentrations, ions, and metabolites, which significantly alter DNA behavior. The chapter discusses experimental methods, highlighting their strengths and limitations in studying DNA under crowded conditions. Key structural transformations—such as B-A and B-Z DNA conformational transitions, triplex formation, G-quadruplex (G4) topologies, and i-motif stabilization—are examined in the context of cosolute-induced effects. The chapter emphasizes the challenges of replicating in vivo conditions in vitro and underscores the need for innovative approaches to study nucleic acids in physiologically relevant environments. These insights advance our understanding of DNA behavior in cellular contexts, with implications for biotechnology, drug design, and gene regulation.