Fly ash derived from coal-fired power stations is the major silica source used for the manufacture of autoclaved aerated concrete (AAC) in the United Kingdom. Typically between 500 000 and 750 000 tonnes of the by-product are used each year. The technical and environmental benefits associated with the use of fly ash contribute to achieving sustainable construction products. This papers briefly reviews the history of the use of fly ash for the manufacture of AAC in the United and discusses the processing methods and relevant specifications. Sources and potential constraints on fly ash supply are noted, as are the on-going efforts to develop complementary sources. Of particular importance for the future manufacture of AAC are the challenges and opportunities of using stockpile and lagoon ashes.