ABSTRACT Tuff beds (volcanic ash beds and tuffs) have been known in the Miocene Monterey and Modelo Formations since they were initially described nearly 100 yr ago. Yet, these tephra layers have remained largely ignored. The ages and correlation of the Monterey and Modelo Formations are predominantly based on associated biostratigraphy. Here, we combined tephrochronology and biostratigraphy to provide more precise numerical age control for eight sedimentary sequences of the Monterey and Modelo Formations from Monterey County to Orange County in California. We correlated 38 tephra beds in the Monterey and Modelo Formations to 26 different dated tephra layers found mainly in nonmarine sequences in Nevada, Idaho, and New Mexico. We also present geochemical data for an additional 19 tephra layers in the Monterey and Modelo Formations, for which there are no known correlative tephra layers, and geochemical data for another 11 previously uncharacterized tephra layers in other areas of western North America. Correlated tephra layers range in age from 16 to 7 Ma; 31 tephra layers erupted from volcanic centers of the Snake River Plain, northern Nevada to eastern Idaho; 13 other tephra layers erupted from the Southern Nevada volcanic field; and the eruptive source is unknown for 12 other tephra layers. These tephra layers provide new time-stratigraphic markers for the Monterey and Modelo Formations and for other marine and nonmarine sequences in western North America. We identified tephra deposits of four supereruptions as much as 1200 km from the eruptive sources: Rainier Mesa (Southern Nevada volcanic field) and Cougar Point Tuff XI, Cougar Point Tuff XIII, and McMullen Creek (all Snake River Plain).