ABSTRACT: The developmental history of North American Land Mammal Ages (NALMAs), their integration with radioisotopic and magnetic polarity dating systems, and relationships to stratigraphic codes and guides are briefly reviewed. Augmented by examples, the need for NALMAs to be supplied with detailed biostratigraphic information is stressed. NALMAs make powerful contributions to the discussion of geohistorical events at intra- and intercontinental scales.