This chapter defines vulnerability in relation to the energy market and reveals that vulnerability – due to timing, structure and nature of the market – is narrower than vulnerability in a societal context. The United Kingdom’s focus is on the affordability of energy, with consumers being considered vulnerable if they spend more than 10% of their income on energy. Vulnerability is important where a consumer is first placed on an energy tariff, which would occur, for example, when the consumer moves to their first home in which they are in charge of the bills. The starting point is to identify that vulnerability in the energy market is a specific type of vulnerability, different from societal vulnerability, but which fits into the category of knowledge and health. In addition, the timing of vulnerability is quite specific in the energy market so that any solution would need to specifically target the moment of entering/renewal of the contract to have the most impact.