Molecular self-assembly is the spontaneous organization of molecules into well-defined, ordered structures or patterns solely through local interactions, occurring without external guidance or manipulation of individual components. As a fundamental concept and research field, it investigates the principles governing how constituent molecules interact via non-covalent forces to create complex, emergent architectures. Its significance lies in providing a bottom-up paradigm for constructing functional materials and nanoscale systems, crucial for advancements in materials science, nanotechnology, and understanding biological organization.