Current trends and policies are progressing in the direction of an increasing share of electricity from renewable energy sources in the EU electricity system, in particular from variable sources such as wind and photovoltaics. In parallel to this, there is increasing use of electric appliances in households, varying significantly throughout the day. Together this results in potentially large and sometimes fast variation of both electricity production and consumption. For a secure energy system, it is critical to maintain continuous service in the face of rapid and large swings in supply or demand. This introductory chapter presents the topics that will be discussed in this section, namely: the need for flexibility and potential solutions; the need for reliability and measuring its cost; and the role of storage in this context.