Interconnections are key to overcoming congestion and bottlenecks in electricity transmission and contribute to the alleviation of variability through two interrelated effects: (1) the technological and geographical spread of variable renewables, and (2) the pooling of conventional capacity (nuclear, hydro, coal and gas) available for flexible dispatch. More centralised market designs can help to improve co-ordination and dispatch efficiency in order to reduce the integration costs of variable renewables. In order to advance towards more integrated systems, it is important to promote common rules and schedules for capacity allocation.