The process of making our electrical energy systems (grids) sustainable is being driven by climate change and security of supply concerns. One pathway to sustainability is to integrate more renewable energy sources, but this can be challenging in particular with variable generation (VG) such as wind and photovoltaics. Sustainable electrical energy systems will also include an increase in demand side measures including the introduction of electric vehicles and demand side participation. The characteristics of VG, in particular wind will be introduced and this will lead naturally into a quantification of their impacts (e.g. thermal plant cycling, reliability and costs) and a description of some solutions to mitigate these. These solution will look at issues such as frequency and voltage control, increasing system flexibility, mitigation of cycling and novel approaches to the utilisation and development of transmission. Some challenges and synergies (with VG), on the demand side will also be explored.