When deciding to deploy high-speed 40 Gbit/s or 100 Gbit/s technologies, operators are faced with an important dilemma in terms of spectral planning: should the new wavelengths be deployed over an existing network, with existing traffic (brownfield deployments), or should they be rolled out over unused or new fibers (greenfield deployments)? While the answer to that question will depend on the context, type of network (metro, long-haul, etc.), capacity requirements, and other factors, operators should keep in mind that each approach has its pros and cons, and particular challenges. This article explores the trade-offs of greenfield and brownfield next generation architectures, the impairments these approaches give rise to, as well as ways to reduce the operational costs that these impairments generate for service providers, based on data from the ITU-T G. 697 standard®. The topic of proper spectral planning and managing capacity will underlay this entire article.