Energy efficient buildings can operate at lower loads, but without optimised systems, they are limited in terms of dynamically responding to grid signals to adjust demand to best meet grid and occupant needs. If enabled with intelligent controls, buildings' efficiency and flexibility features can become resources with value in electricity markets. As a result, grid-interactive efficient buildings could reduce and shift electricity consumption to the mutual economic benefit of building owners and grid operators, relieve system stress, better integrate variable renewable energy generation, and better meet building occupant expectations for optimised comfort, cost, and resilience. This paper describes how a new class of grid-interactive efficient buildings (GEBs) can both reduce net demand and benefit the grid through more flexible loads. This research provides a new taxonomy for flexible building technologies: flexible timing, flexible efficiency, flexible fuel source, and flexible frequency/voltage regulation. Each of these flexibilities is linked to grid services recognised by electricity markets and analysed in terms of technical potential and optimisation requirements. This paper demonstrates an estimated 1.7 exajoules (XJ) of summer season energy savings from the technical potential of efficiency and flexibility in building cooling equipment. By targeting high-value electricity, these strategies could bring an estimated €28 billion in cost savings and over 60 million metric tons (Mt) in CO2 emission savings.