Aggregates containing certain constituents can react with alkali hydroxides in concrete. The reactivity is potentially harmful only when it produces significant expansion. This alkali-aggregate reactivity (AAR) has two forms--alkali-silica reaction (ASR) and alkali-carbonate reaction (ACR, sometimes called alkali-carbonate rock reaction, or ACRR). ASR is of more concern than ACR because the occurrence of aggregates containing reactive silica minerals is more common. Alkali-reactive carbonate aggregates have a specific composition that is not very common. Alkali-silica reactivity has been recognized as a potential source of distress in concrete since the later 1930s. Even though potentially reactive aggregates exist throughout North America, ASR distress in structural concrete is not common. There are a number of reasons for this: 1. Most aggregates are chemically stable in hydraulic-cement concrete 2. Aggregates with good service records are abundant in many areas 3. The concrete in service is dry enough to inhibit ASR 4. The use of certain pozzolans or slags controls ASR 5. In many concrete mixtures, the alkali content of the concrete is low enough to control harmful ASR 6. Some forms of ASR do not produce significant deleterious expansion To reduce ASR potential requires understanding the ASR mechanism; properly using tests to identify potentially reactive aggregates; and, if needed, taking steps to minimize the potential for expansion and related cracking. Alkali-carbonate reaction in concrete was not documented until 1957. Although ACR is much less common, this report also briefly reviews the mechanism, visual distress symptoms, identification tests, and control measures.