Many geologic formations in the United States yield pozzolans or materials which can be converted into pozzolans by grinding and calcination. When properly selected, processed, and used, pozzolans will reduce costs, improve quality of concrete, protect concrete against effects of reaction between aggregate and cement alkalies, and inhibit attack by aggressive waters. With improved knowledge of their properties and sources, use of pozzolans is increasing. The geologist and petrographer can aid materially in the search for and exploitation of deposits, and in selection of processing methods.Natural pozzolans owe their activity to five substances, namely: volcanic glass, opal, clays, zeolites, and hydrated aluminum oxides. Each active substance contributes characteristic properties to the pozzolan; consequently, the quality of proposed materials and need for special processing usually can be predicted from petrographic analyses.Geologic formations known to yield pozzolans are indicated, and data on 95 samples representative of tested formations are summarized. Properties and response to heat treatment of pozzolans of the various activity types are discussed.