The true test of how a vendor's systems work is how they work with the service provider's existing systems and with other vendors' systems in operating networks. In this paper, we present various concepts, including design for X (DFX), and introduce the concept of design for networks (DFN), the objective of which is a working, revenue-generating network that meets or exceeds the customer's expectations. In a series of case studies involving optical networks, we show what can go wrong when DFN considerations are not part of the product, network, and service design process. We also discuss such network considerations as synchronization, data communications networks (DCNs), management, and synchronous optical network (SONET)/synchronous digital hierarchy (SDH) interfacing, and we introduce an expansive definition of product and design. Finally, we show how to transform the lessons learned in the field into feedback for the design process so as to improve the end result for both vendor and customer.