The development of CPS involves multiple stakeholders which have different viewpoints and therefore use different concepts, models and tools to deal with their concerns of interest. Successful development requires that relations between viewpoints are addressed. This lecture identifies that such relations exist at three levels: people, models and tools. We present solutions that formally and explicitly capture these relations at each of these three levels. Interrelation contracts are used to define the vocabulary, assumptions and constraints required for ensuring smooth communication between stakeholders (people level). Dependency models capture relations between product properties belonging to different viewpoints, and how such dependencies relate to predictions and decisions (model level). Tool integration models describe the relations between tools in terms of traceability, data exchange, invocation and notifications (tool level). An industrial robot case study is utilized to illustrate the challenges and solutions with respect to relations between viewpoints. We also provide an unification approach discussing how these solutions complement and can be used to support each-other.