This digest covers the main technical issues and answers the frequently asked questions on the specification, application and performance of AAC masonry in the light of current regulations and guidance. It also looks forward to the expected future position and evaluates the green credentials on the basis of available data on energy and resource usage published by the manufacturing industry. This Digest is in two parts; figure, table and reference numbering continues into Part 2 which contains the full list of references, documents mentioned in the text, and further reading. Part 1 covers standards and codes for autoclaved aerated concrete blocks, their composition and manufacture, physical properties (density, strength, shrinkage, water absorption, resistance to freeze-thaw) and durability. 8 pages. Digest 342 is now withdrawn.