As the United States moves towards a more electrified clean energy future, the need for transmission infrastructure enhancement and expansion is paramount. The combination of an aging grid, increasing electricity demand, increased frequency of extreme weather events, the need to connect renewable energy resources with demand, and the push toward electrification of both buildings and transportation are contributing to the growing need for transmission improvements and expansion. Transmission planning can take various forms: planning for transmission expansion and building of new transmission lines, increasing the efficiency, resilience, and capacity of existing transmission lines, as well as deferring transmission needs through non-wires alternatives. State Energy Offices can play a wide spectrum of roles in transmission planning, either directly or indirectly, from developing policy goals, engaging in state-level energy planning, conducting modeling and analyses, and convening relevant stakeholders.