Critical energy infrastructure has to operate against extreme events, like long-term climate change, natural hazards and disasters, cyber intrusion, etc. Fast recovery and successfully adapting to extreme events are critical to build a resilient power grid. The rapid development of smart grid technologies and the increasing penetration level of distributed energy resources (DER) provide opportunities to reevaluate the restoration process of transmission and distribution (T&D) systems, through appropriate modeling and simulation of T&D systems and their interactions. Traditionally, power system restoration studies simply treated distribution networks (DNs) as load injections into buses, without considering power flows or DER capabilities inside DNs. When performing the transmission restoration, the boundary substation buses are assumed not to be affected by the actual pick-up load capability of DNs. This assumption makes the restoration of bulk power system easier, but fails to reflect the actual operation of power systems and may cause inaccurate planning solutions. On the other hand, emerging smart grid technologies provide capabilities for deploying distribution management systems (DMS) and advanced communication infrastructures, which enables the capability of integrating T&D system restoration.