This book concerns aggregates in concrete. It deals not primarily with aggregates per se, but with the role that aggregates play in producing this construction material. Its theme is the importance of aggregates in modern concrete science and engineering. The book argues for a better understanding and appreciation of the role of aggregates, and illustrates how aggregates crucially influence the properties of the composite material. It departs from the outdated view of aggregates as being simply inert fillers, little more than a bulk constituent required for mass and economy. A materials science view is taken in which each constituent of the concrete is important in its own right, with interaction between the constituents governing the overall properties.