Autoclaved aerated concrete (AAC) as a building material has been industrially produced since the beginning of 20th century. It is a form of concrete with very lightweight, obtained by uniformly distributed, closed air bubbles. Its thermal conductivity is one-sixth or less that of concrete, so it has the ability to provide cost-effective design solutions when used in low-energy buildings. This chapter explains how AAC is manufactured and gives the basic characteristics of the material. AAC can be made in a number of different products, although throughout the globe, it is generally used as masonry. There is, however, a growing market for reinforced products used in walls, floors, and roofs. Various applications of the products are demonstrated as well as some design examples when used in the United States, including seismic design.