The supply of aggregates suitable for use in construction and maintenance of infrastructure in western North America is a continuing concern to the engineering and resources-management community. Steady population growth throughout the region has fueled demand for high-quality aggregates, in the face of rapid depletion of existing aggregate resources and slow and difficult permitting of new sources of traditional aggregate types. In addition to these challenges, the requirement for aggregates to meet various engineering standards continues to increase. In addition to their physical-mechanical properties, other performance characteristics of construction aggregates specifically depend on their mineralogy and texture. These properties can result in deleterious chemical reactions when aggregate is used in concrete mixes. When this chemical reaction-termed 'alkali-aggregate reaction' (AAR)-occurs, it can pose a major problem for concrete structures, reducing their service life and requiring expensive repair or even replacement of the concrete. AAR is thus to be avoided in order to promote the longevity of concrete structures and to ensure that public moneys invested in infrastructure are well spent. Because the AAR phenomenon is directly related to the mineral composition, texture, and petrogenesis of the rock particles that make up aggregates, an understanding of the relation between the geology and the performance of aggregates in concrete is important. In the Pacific Northwest, some aggregates have a moderate to high AAR potential, but many others have no or only a low AAR potential. Overall, AAR is not as widespread or serious a problem in the Pacific Northwest as in other regions of North America. The identification of reactive aggregates in the Pacific Northwest and the accurate prediction of their behavior in concrete continue to present challenges for the assessment and management of geologic resources to the owners and operators of pits and quarries and to the users of the concrete aggregates mined from these deposits. This situation is complicated by the length of time typically required for AAR to become noticeable in concrete construction in the Pacific Northwest, commonly on such a scale that other deterioration mechanisms may have masked the effects of AAR. Distinguishing between the effects of AAR and those related to other problems in concrete is important for understanding the nature and severity of AAR throughout the Pacific Northwest. Furthermore, developing an understanding of the extent of the problem will assist efforts to maximize the intelligent and stewardly use of aggregate resources in the Pacific Northwest. This chapter illustrates the current 'state of the art' of AAR studies in the Pacific Northwest, a region with a common geologic heritage as well as many distinct geologic elements. The optimal use of aggregates in the construction of concrete structures that will achieve their design life is possible through an understanding of the engineering and geologic properties of these aggregates and of their geologic setting.