The paper discusses target space duality in string theory, focusing on discrete symmetries and their implications for the moduli space of toroidal and other compactifications. It reviews the concept of duality, which allows for the interchange of different string backgrounds, and explores its manifestations in various settings, including flat compactifications, orbifold and Calabi-Yau backgrounds, and curved backgrounds with Abelian symmetries. The paper emphasizes the role of duality in relating different string theories and in preserving symmetries in effective field theories. It also discusses the relationship between duality and gauge symmetries, and how duality can be used to understand the behavior of strings in different backgrounds. The paper concludes with a discussion of the implications of duality for the low-energy effective action of the heterotic string and the relationship between duality and the electric-magnetic duality of four-dimensional vector fields. The review highlights the importance of duality in string theory and its potential to provide insights into the underlying structure of the theory.The paper discusses target space duality in string theory, focusing on discrete symmetries and their implications for the moduli space of toroidal and other compactifications. It reviews the concept of duality, which allows for the interchange of different string backgrounds, and explores its manifestations in various settings, including flat compactifications, orbifold and Calabi-Yau backgrounds, and curved backgrounds with Abelian symmetries. The paper emphasizes the role of duality in relating different string theories and in preserving symmetries in effective field theories. It also discusses the relationship between duality and gauge symmetries, and how duality can be used to understand the behavior of strings in different backgrounds. The paper concludes with a discussion of the implications of duality for the low-energy effective action of the heterotic string and the relationship between duality and the electric-magnetic duality of four-dimensional vector fields. The review highlights the importance of duality in string theory and its potential to provide insights into the underlying structure of the theory.