This chapter describes the architectural style of mixed-criticality systems and introduces the DREAMS waistline structure of services. It provides structuring rules according to several integration levels to support the integration of applications with different timing models and different safety assurance levels. Section 2.1 introduces the system model in mixed-criticality systems and describes the logical and physical system structures. In addition, it shows how those two system models can be mapped by a mixed-criticality name space. Section 2.2 elaborates on the DREAMS waistline structure of services and lists the core services as well as the optional services. In addi tion, the architectural building blocks and services of communication services, global time services, execution services as well as local and global resource management services are elaborated in this section. Section 2.3 gives an overview of the model-driven development based on the described building blocks. The DREAMS certification strategy along with the concept of modular certification and mixed-criticality patterns are introduced in Section 2.4. Fault assumptions is another topic which is covered in Section 2.5. The concept of fault containment regions is introduced to overcome the design faults as well as the physical faults. In addition, the failure modes, threats, threat models and threat analysis for the four major core services are elaborated in this chapter. At the end, the DREAMS harmonized platform as instantiation of the DREAMS architectural style is introduced in Section 2.6.