The worldwide rapid urbanization and population growth have heightened the demand for fast and efficient construction techniques. Among the emerging trends in the construction industry, modular systems manufactured off-site and transported for on-site assembly using joint connections have garnered significant attention. This paper provides a comprehensive review of modular structures, addressing general concepts such as structural modular units, types and classification of modules, definition of hybrid systems, types of lateral resisting systems, and modular connections. The paper identifies challenges in modular construction, particularly the lack of design guidelines, difficulties associated with handling and transportation, and insufficient knowledge regarding their seismic performance. Special focus is dedicated to the design and seismic behaviour of steel modular systems. The paper includes a case study of a modular building designed according to the current specifications of Eurocode 8. The suitability of traditional design approaches is discussed, and specific research needs are identified.