These sources comprehensively outline refactoring, the systematic improvement of code without altering its functionality, emphasizing its importance in combating technical debt, which arises from poor coding practices and rushed development. The materials detail code smells, indicators of underlying problems in code categorized as bloaters, object-orientation abusers, change preventers, dispensables, and couplers, and provide numerous examples of each. Furthermore, the resources present various refactoring techniques, categorized by composing methods, moving features, organising data, simplifying conditionals, simplifying method calls, and dealing with generalisation, illustrating their application with before-and-after code examples. Finally, the sources discuss when and how to refactor effectively, including the use of automated tools for detecting code smells and the critical role of testing throughout the refactoring process to ensure code reliability.