Mono Basin is a post-Miocene, northeast-trending structural depression lying immediately east of the central Sierra Nevada. A negative gravity anomaly of about 50 milligals centers under Mono Lake in the western portion of the basin, which is surrounded by varied and voluminous volcanic rocks of Pliocene, Pleistocene, and Holocene age, and has been regarded by others as a volcano-tectonic depression. K-Ar ages and the distribution and relations of the young volcanic rocks show that they represent several separate volcanic episodes and that volcanism was synchronous around the basin only during the period between about 2 and 4 m.y. ago. Structural development of the present depression has occurred largely during the last 3 m.y. and is still in progress. Structures around the margins of Mono Basin suggest that it is a broad, probably shallow, warp with an axis that plunges gently toward the southwest where it terminates against the large dip-slip fault along the eastern front of the Sierra Nevada. The throw on this fault has been about 6000 feet, and the escarpment produced is now about half buried by basin fill. The gravity anomaly is probably produced by very low-density pumiceous fill rather than by extreme depth. The northeasterly trend of the basin is a reflection of a regional zone of northeasterly structural trends, extending far east of Mono Basin itself, within which left-lateral movements have occurred. The junction of this zone with northwest-trending structures to the south and west produces a structural “knee,” a zone of extension along which voluminous Pliocene-Pleistocene volcanic eruptions have occurred east of Mono Basin. The deepest part of the basin, under Mono Lake, is at the western end of this structural “knee.” Mono Basin is not a caldera-like collapse structure due to eruption of magma. The total estimated volume of lava erupted during Pliocene and Pleistocene time in the region around the basin has been only about one quarter of the volume of the present depression. Volcanism and deformation have been synchronous, and their manifestations are interrelated, but the relation appears to be incidental rather than necessary. Deformation appears to have been approximately continuous during the last 3 to 4 m.y. Rates of differential vertical movements in the marginal regions are estimated at 1 to 2 feet per thousand years, and between the margins and center of the basin to have been between 2 and 4 feet per thousand years.