This is an introductory module of the lectures corresponding to the Efficient Building Operation discipline of the SINERGY project and will provide an overview of the landscape around the subject of Building Efficient Operation and supporting disciplines. Buildings represent a significant sector in the future low-carbon economy, with nearly one third of the final energy use in developed countries. Policy responses to the efficiency challenge have put pressure in the design and operation of buildings. They emphasise the adoption of renewable energy sources, the integration of recent smart technologies and enhancement of thermal and operational efficiency. Lately, user behaviour and improved levels of quality have also become a part of the overall pursuit of more efficient buildings. It is therefore paramount to understand the complex interactions of several influencing factors: buildings physics and thermodynamics, the ever-changing technologies supporting building operation, the user interactions (feedback and comfort), and make them compatible with the decarbonisation agenda.