Ion exchange is the selective exchange of ions between a solid and liquid, enabling targeted removal or recovery. Ion exchangers are key materials in purification and resource recovery applications. This chapter provides a detailed introduction to ion exchange materials, exploring their fundamental principles, types, and practical applications. It covers natural, synthetic, hybrid, and eco-friendly ion exchangers, discussing their structure, properties, and the factors that influence their performance, such as porosity, cross-linking, and ion selectivity. The chapter also examines the thermodynamic and kinetic aspects of ion exchange processes, along with the mechanical and chemical stability required for their use in challenging environments. This chapter highlights key applications, including water purification, heavy metal removal, lithium recovery, pollutant detection, and radioactive waste treatment. It sets the foundation for understanding the vital role of ion exchange materials in various environmental and industrial contexts, with subsequent chapters providing a more detailed examination of these applications.