Retrofit typically includes other works, including essential maintenance and improvements to the aesthetic appearance of the building, which are carried out at the same time. Instead retrofit aims to upgrade an existing building to meet wider objectives: reducing energy consumption and improving thermal comfort. The introduction of the Minimum Energy Efficiency Standard in March 2018 is the first step in enforcing thermal improvements in the worst-performing parts of the building sector and will require building owners, both public and private, to achieve a minimum Band E performance. Numerous studies of completed Passivhaus/EnerPHit buildings have shown that significant improvement in the building fabric and resulting reduction in both heat losses and overheating risk can dramatically reduce energy costs and improve thermal comfort. Unlike newbuild projects, retrofit schemes benchmark against both existing energy performance and existing thermal comfort. The chapter also presents an overview of the key concepts discussed in this book.