This book focuses on the nature and performance of aggregates in concrete. It emphasizes the interaction between matrix and aggregate phases, and how this interaction governs properties of the composite material. Before discussing the properties of aggregates themselves and their performance in concrete, we need to consider the origin and sources of aggregates, and how they become useful engineering materials. The subject matter in this and the subsequent four chapters will cover mainly conventional (normal strength) concretes and natural aggregates. Such aggregates may be defined as materials composed of rock fragments which are used in their natural state except for such operations as crushing, washing, and sizing. Natural aggregates are derived from naturally occurring geological sources, which are processed and beneficiated to a greater or lesser extent to produce hard, non-cohesive granular materials of varying sizes that can be incorporated into concrete. Natural aggregates are processed by crushing, screening, and washing to render them useful for engineering purposes. They exclude synthetic or artificial aggregates or marginal aggregates which are covered in Chapter 7. Also included in this chapter are aggregates that require minimal processing, and may be usable virtually from source, such as certain sands and gravels.