A rapidly growing strategy in product design and manufacture, with great potential to improve customer value, is mass-customization. The main idea is to divide the product into modules that can be shared among different product variants. This will support a wide range of options for the end customer to select among, while an internal efficiency, similar to mass-production, can be achieved. This has been a success for many companies acting on the consumer market. However, many manufacturing companies are engineer-to-order (ETO) oriented, such as original equipment suppliers (OES). They design a unique solution, often in close collaboration with other companies. The solution can then be manufactured in different quantities depending on the client’s need. For these companies, there is a strategic need for developing high quality engineering support to further utilize and exploit the information and knowledge produced during product development and to succeed with a strategy influenced by the principles of mass-customization. This has to include the implementation and management of systems enabling highly custom-engineered products to be efficiently designed and manufactured. One challenge when introducing such flexible support is to enable traceability of decisions taken, tasks executed, knowledge used and artefacts developed throughout the whole lifecycle of an individual product. In this chapter, it is shown that traceability can be achieved by introducing support for capturing, structuring and mapping between decisions and resulting outputs, such as geometrical building blocks, knowledge implemented as rules, and the argumentation for the selection, design and specification of these. Three examples are presented where the concept Design Description has been modelled based on an item-oriented, a task-oriented, and a decision-oriented perspective which show the generality of the Design Description concept. The three examples demonstrate how to use the Design Description to enable traceability in platform design, product design, and manufacturing development processes.