The research carried out in this PhD thesis focuses on the integration phenomenon in the design of mechatronic products. It contributes to the understanding of the phenomenon and to conceptual modelling of mechatronic products. These products are perceived to have emerged as a consequence of a joint effort between mechanical, electronics and software development. Companies engaged in multi-disciplinary product development face the challenge of coordinating different engineering disciplines to pursue a synergistic effect and to avoid failures when solutions from the different engineering disciplines have to be combined and function in an integrated fashion. The holistic view of the product concept is sometimes missing in projects and engineering disciplines are reluctant to interact. Investigations of literature, cases from industry and practical experience are used for dissecting the integration phenomenon. This reveals a vast number of challenges, which companies experience in relation to the phenomenon, ranging from product-level challenges to organization-level challenges. A comprehensive literature study is carried out with the aims of finding proposed solutions for addressing the challenges related to the integration phenomenon and evaluating how well the challenges are addressed by using the solutions in a design setting. The study shows that the available solutions only partly cover the stated challenges and that a large part of the identified solutions appear to support analysis activities rather than synthesis activities. The study further shows that functional modelling can be used to create a common model in the early phases of design, but later phases have to rely on informal and formal transformation of information about the product concept between the domains due to the absence of a common modelling language. The further research is aimed at one particular challenge related to the integration phenomenon, which is the difficulty in modelling and controlling multiple relations in the product concept as a consequence of the multidisciplinary development. ‘A product-related dependency’ is defined to describe relations in the product concept between functions, properties and means (solutions) appearing as a consequence of the design process. Based on three cases from industry a classification of dependencies is established, highlighting thirteen types of generic dependencies to be aware of when designing mechatronic products. The literature study shows that such a classification has not been attempted before. Without a classification to provide a basis for a structured search it is left to chance for the development team to discover potentially critical dependencies in due time. A ‘Mechatronic Integration Concept’ is proposed aiming at operationalizing the use of the classification. The purpose is to facilitate discussions between engineers from different disciplines on matters that are important to clarify between the designers working on contributing with solutions to the product concept. Hence, it is not meant to be a conceptual description of every important aspect of the product concept. The classification and the Mechatronic Integration Concept are evaluated by deploying them in a mechatronic project from industry. The results from the evaluation show that the thirteen groups of dependencies can be identified, clarified and modelled by use of the classification and the Mechatronic Integration Concept. The results also indicate that rework can be avoided, leadtime can be shortened and the performance of the product increased by utilizing them in industrial projects.