Mesosiderites are very intimately mixed stony-iron breccias, including little recrystallized and highly recrystallized varieties (I-III) and a subgroup (IV) with melted matrix. All mesosiderite breccias contained some material that was hot, as indicated by pigeonite that inverted at 1150¿C. Subgroup IV contained silicate and metal liquids at temperatures of at least 1300¿C and 1350¿C, respectively. Cooling rates are not rigorously defined, but are approximately 1¿/year in the high temperature range and 1¿/m.y. below 500¿C. The latter cooling requires a depth of 40 km in an asteroid with a radius of 400 km, or less if porous, low-conductivity of 400 km, or less if porous, low-conductivity surface material is present. Cratering with metal projectiles appears incapable of generating mesosiderites: a normal impact produces rapidly cooled, metal-poor breccias. Impacts on a crust in which metal-silicate segregation had taken place could generate stony-iron breccias, but burial under an insulating blanket would be necessary to obtain slow cooling. Disruption and reassembly of two colliding bodies and accreation of differentiated planetesimals could form stony-iron breccias at depth but, as with the previous two models, the easiest way to explain the low olivine of mesosiderites is to postulate derivation from olivine-poor chondritic material. Two internal processes, turbulent metal-crust mixing during core formation and crustal blocks sinking into the core, provide a deep location and withhold olivine from the breccia. However, these mechanisms do not explain how the silicate and metal and metal liquids in subgroup IV (otherwise interpreted as impact melt-rocks) could have failed to segregate in a deep, slowly cooled environment. No single model for mesosiderite origins is wholly satisfactory.