The Bureau of Reclamation has had a strong interest in the development and application of concrete technology to the design, construction, and maintenance of water resources development projects. The application of petrographic methods to concrete technology has been a natural outgrowth of Reclamation's involvement in the development of concrete technology. Reclamation's first petrographer was W. Y. Holland who, purely out of curiosity, employed petrographic techniques to see what types of rocks were failing in the sulfate soundness test for aggregates. As this provided useful information for the evaluation of the suitability of aggregates, Reclamation began to routinely perform petrographic examination of aggregate samples to provide information on the suitability of aggregates and to compare samples from different sources. The method was also used to examine concrete to identify rock types failing in the freeze-thaw test, and to determine the cause of distress in deteriorated concrete. The petrographic examination received its biggest impetus in the early 1940's in the investigations of alkali-aggregate reaction in concrete and the development of precautionary measures. Reclamation established its Petrographic Laboratory in 1941 to investigate deleterious chemical reactions in concrete. The petrographic method has become widely accepted and is now routinely applied in concrete technology investigations throughout the world. The main applications of petrographic methods are in the evaluation of concrete aggregates for use in concrete, the assessment of the quality of concrete and the diagnosis of the cause of distress in deteriorated concrete, and as a basic procedure in concrete research investigations.