Lunar mare basalt deposits cover 17% of the lunar surface, occur preferentially in topographic lows on the nearside, and have a total volume estimated at 1 × 10 7 km 3. Analysis of returned samples and photogeologic and remote sensing studies shows that mare volcanism began prior to the end of heavy bombardment (the period of cryptomare formation), in pre-Nectarian times, and continued until the Copernican Period, a total duration approaching 3.5-4 Ga. Stratigraphic analyses show that the flux was not constant, but peaked in early lunar history, during the Imbrian Period (which spans the period 3.85-3.2 Ga). Average volcanic output rate during this peak period was about 10 -2 km 3/a, very low relative to the present global terrestrial volcanic output rate and comparable to the present local output rates for individual volcanoes such as Vesuvius and Kilauea, Hawaii. Volcanic landforms indicate that many eruptions were of high volume and long duration. Some eruptions associated with sinuous rules may have lasted of the order of a year and emplaced 10 3 km 3 of lava, representing the equivalent in one year of about 70,000 years at the average flux. Shallow magma chambers were uncommon. The nearsidefarside mare deposit asymmetry can be readily explained by differences in crustal thickness. Magma ascending from the mantle or from a buoyancy trap at the base of the crust should preferentially extrude to the surface on the nearside, but should generally stall and cool in dikes in the farside crust, extruding only in the deepest basins. The occurrence of farside maria within craters whose diameter is generally near to or less than the thickness of the crust may be accounted for by the difference between local and regional compensation. Dikes that establish pathways to the surface on the nearside should have very high volumes, comparable to the volumes associated with many observed flows and sinuous rilles. An abundance of dikes should exist in the lower crust of the Moon, many more than those feeding surface eruptions (the upper limit is 37-50% of the crust by volume). The presence and abundance of such dike swarms have important implications for the interpretation of the average composition of the lunar crust and the composition of basin and crater ejecta. The interplay between thermal contraction and differentiation leads to net cooling and ultimate contraction of the outer portions of the Moon, resulting in a regional horizontal compressive stress acting on the lunar crust. In addition, overall cooling and deepening of sources require the production of ever larger volumes of magma in order to reach the surface. With time, stresses became high enough so that few dikes could open to the surface, causing eruptive activity to be severely diminished in the Eratosthenian, and perhaps to cease in the Copernican Period. Lower stress levels are required to terminate eruptive activity on the lunar farside, consistent with the Imbrian age of the farside maria and the nearside location of the youngest maria. Lunar mare deposits provide an example of the transition from primary crusts to secondary crusts and illustrate the significance of several factors in the evolution of secondary crusts, such as crustal density, variations in crustal thickness, presence of impact basins, state and magnitude of stress in the lithosphere, and general thermal evolution. These factors are also responsible for the extremely low volcanic flux, even during periods of peak extrusion.