The interface between the product development function and the manufacturing function is one key dimension in new product development (NPD) projects. Hard and soft requirements for manufacturability are defined and communicated to product development teams early in the NPD project to ensure the new products are fit for the manufacturing system. In this chapter, we determined what requirements are important for a manufacturing system and how these requirements are handled during an NPD project by analyzing two industrial cases. The results showed that requirements communicated from the manufacturing function to the design function had different sources and effects on different aspects of the manufacturing system. They were communicated and integrated to various degrees and through various mechanisms. There was a tendency to rely on the personal and verbal communication of requirements, as opposed to using more formal structured methods. This way of working was sufficient when product change was incremental and not radical. The case studies showed that the manufacturing function needed to employ more efficient methods to define and communicate their requirements in large and complex NPD projects.