Walls constructed with an air space within the masonry, termed hollow or cavity walls, are commonly used in modern building construction as a means of protecting the wall against rain passing through it, and enhancing the thermal insulation of the wall. Such walls are widely used in Great Britain but are also a part of modern building techniques in North America, where the cavity is sometimes filled with insulating material. In the history of the use of hollow masonry walls, which extends back at least to the early decades of the 19th century, various methods have been employed for their construction. The purpose of these notes is to review the development of hollow wall construction by reference to the literature, as well as by reference to certain hollow wall constructions revealed in the alteration and demolition of old buildings.