Switch 2.0 is an open-source platform for planning transitions to low-emission electric power systems, designed to satisfy 21st century grid planning requirements. Switch is capable of long-and short-term planning of investments and operations with conventional or smart grids, integrating large shares of renewable power, storage and/or demand response. Applications include integrated resource planning, investment planning, economic and policy analyses as well as basic research. Switch 2.0 includes a complete suite of power system elements, including unit commitment, part-load efficiency, planning and operating reserves, fuel supply curves, storage, hydroelectric networks, policy constraints and demand response. It uses a robust, modular architecture that allows users to compose models by choosing built-in modulesà la carte or writing custom modules.