This contribution outlines the creation and use of system models for the successful and efficient design and development of multidisciplinary mechatronic systems. As functionality increases, so the complexity of mechatronic systems increases. Furthermore, a variety of engineering disciplines are involved in the development of these systems, which makes the whole design process even more complex. Handling such development tasks and their complexity requires the kind of holistic view of the whole system and basic understanding of the main design parameters and their mutual influences that can be achieved using system models. This paper reviews the literature on system models and presents fundamental principles and methodologies (i.e., 'top-down' and 'bottom-up' approaches) for the creation of system models in mechatronic design. Two industrial examples from the field of continuous casting are used to show the advantages of system models.