The following chapter introduces product structure, function structure, and product architecture as complementary views shaping sustainability across the life cycle. Product structure hierarchically organizes assemblies and parts, influencing procurement, manufacturing, service, and end-of-life. Function structure decomposes the overall function into sub- and auxiliary functions, while product architecture links both, mapping functions to physical elements to define platforms, modules, and construction kits. Modularisation is presented as a central strategy: decoupled, testable modules enable independent development, reuse across variants, and scalable product families. The chapter contrasts integral construction with composite, highlighting sustainability trade-offs between efficiency, serviceability, and recyclability. Platform strategies standardize shared modules across variants and generations to balance variety, cost, complexity, and environmental impact. Building on architecture, the 9R framework structures circular strategies: R0–R2 (refuse, rethink, reduce) minimize resource demand; R3–R7 (reuse to repurpose) extend service life; R8–R9 (recycle, recover) reclaim materials or energy when higher options fail. An example illustrates levers across all R-levels, from minimalist features and sharing models to repairability and material recycling. The chapter concludes that circular performance depends on early architectural choices that enable high-priority R-paths and align product families with sustainable value chains.