In this survey, we provide an overview of model order reduction (MOR) methods applied to coupled systems. The coupling can be linear or nonlinear, weak or strong. Physically, the coupling may come from structure-structure coupling, fluid-structure or fluid-fluid interaction, electro-thermal coupling, electro-or thermal-mechanical coupling, or circuit-device coupling. Different MOR methods for coupled systems are reviewed, and numerous examples of coupled systems from the open literature are used for illustration.