What is embodied carbon and why is it a significant challenge for clients and designers in the real estate and construction sector? It is the sum of greenhouse gas (GHG) emissions that arise in the life cycle of a building during manufacture and construction (upfront), maintenance and replacement of building components (recurrent), as well as dismantling and waste processing (end of life). Currently, the relative and absolute share of embodied carbon in the life cycle of a single building is growing and becoming a dominant factor in the case of energy-efficient buildings. For example, for new buildings, it can represent more than 50% of life-cycle carbon. Against this background, embodied carbon is becoming an object of assessment not just in research but also in design and decision-making. It also becomes a key action to reduce GHG emissions. Embodied carbon assessment and reduction are being increasingly mandated in national regulations. Clients and designers (as key actors in the supply chain) can harness new knowledge and tools to reduce embodied carbon as part of a strategy to reduce overall GHG emissions. Appropriate methods, data, benchmarks and tools are being further developed and operationalised to support the processes for specifying and designing low carbon buildings. An overview is presented of the state of knowledge and current developments. Constructive recommendations are provided for actions that clients and designers can take.