Momentous changes are occurring in mission-critical communications networks. The current SDH/SONET networks, which are TDM-based, have proven to be resilient, reliable and secure in delivering voice, video and data traffic 24x7. However, there are a number of challenges driving many operators of mission-critical communications networks to consider migrating to IP/MPLS networks: the imminent obsolescence of SDH/SONET network equipment, the approaching end of their TDM leased line contracts with service providers, the adoption of new bandwidth-intensive applications and the pressing need to consolidate multiple dedicated networks into one converged network to optimize costs. This paper discusses the key considerations for network migration planning and provides an SDH/SONET-to-IP/MPLS migration blueprint.