This chapter sets the scene for the book by highlighting that "energy transition" is more than a catchphrase. Rather, the transformation of our energy system is an essential requirement to comply with international agreements and EU ambitions to curb climate change. It is also a reality that can already be witnessed today. This chapter outlines some key features of this transition focusing on energy supply in general, and more specifically on the electricity, heating and cooling, and transport sectors. The transition is however not just limited to these energy sectors, but will also affect the role of consumers, who are expected to form a much more central part of the energy system of the future. In addition to society, the changes foreseen will naturally also affect our economy, both regarding the required investments, but more importantly regarding the novel market designs that are needed to trigger these investments, while ensuring cost-competitiveness and system reliability. Numerous challenges are therefore ahead of us, challenges policy making in Europe will need to address.