Over the last 18 months Software-Defined Wide-Area Networking (SD-WAN) has emerged as a new technique for building and operating enterprise IP networks. Based on implementing network functions such as routing and security as virtualized processes, SD-WAN can provide increased flexibility and agility for enterprise networks to better support new cloud-centric application architectures. In some cases, SD-WAN can reduce operating costs as well. This paper reviews the top functional, technical, and economic drivers of SD-WANs.