Sustainable smart cities need to focus on energy production and use. By installing solar panels, prosumers may contribute to the energy production in the city. The use of solar panels is particularly relevant to free-standing residential buildings. Prosumers may also trade flexibility, the ability to shift energy use to periods when the total energy consumption is lower. Prosumers may also store energy for future sale or consumption. An aggregator is a new role connecting prosumers with energy providers. The aggregator negotiates terms, provides flexibility on behalf of its prosumers, and may even provide energy storage capabilities. This chapter describes the evolving role of aggregators and their possible business models. The aggregators will contribute to smarter energy production and use in smart cities.