This monograph is concerned with lightweight aggregates that can be used in structural lightweight concrete; namely those produced from clay, shale, slate, blast furnace slag, and fly ash. The clays, shales and slates are the most widespread of raw materials, although slate is not used in Canada. All the producers in Canada use the rotary kiln method, but sintering is also used in the United States. The production of expanded blast furnace slag is restricted to locations of steel plants producing pig iron in blast furnaces. There is only one plant in Canadaproducing expanded slag lightweight aggregate. Fly ash is not used in Canada but is to a limited extent in the United States.A machine process was developed in Canada by which a pelletized expanded slag could be produced. This process almost completely eliminates the sulphurous fumes associated with the widely used pit process. Several of the machines have been installed in slag-expanding plants in various countries.The lightweight aggregates being produced are all used in concrete masonry units, for both load-bearing and non-loadbearing applications. The lightweight aggregates produced from clays and shales and, to a limited extent, from expanded slag are used in structural cast-in-place and precast concrete in beams, columns, walls, roofs, and floors in low- and high-rise structures and in bridges.Lightweight concrete is about 30 per cent lower in unit weight than normal weight concrete and is lower in thermal conductivity and sound transmission. Although the properties of the various lightweight concretes differ, they are similar in many respects to normal weight concrete. Each aggregate and the concrete made with it must be assessed on their own properties.ASTM standards have been adopted in Canada to limit the properties of the lightweight aggregates and the concretes made with them. Although the cost of lightweight aggregates is higher than that of normal wéight aggregate, appreciable savings have resulted from their use in many applications, primarily because of the lower unit weight.The lightweight aggregates have other, non-structural, applications such as back fill, floor fill, and soil conditioning. In the case of expanded slag, it is finding a use in the manufacture of portland-blast furnace slag cement.