Over 7 trillion barrels of viscous oil (heavy oil, extra heavy oil, and bitumen) are trapped in sandstones or unconsolidated sand formations around the world, mainly in Canada, Venezuela, and Russia. To date, only cold flow methods and steam injection processes have achieved commercial success in accessing this immense resource. This article highlights the definitions, geology and origins, and geographical distribution of the viscous oil resources in the world, then describes the approaches and physical mechanisms of the major commercialized viscous oil production methods being practiced around the world. Approaches to calculating thermally-induced stresses are presented, as well as discussions of thermo-mechanical issues associated with commercial thermal processes. Then, thermal, physical, and geomechanical properties of sandstone under high temperature and pressure are investigated, based on field and laboratory data. An important factor is the change in rock properties that takes place because of the large thermally-induced stresses. Finally, a practical example of thermal geomechanics effects during thermal oil production operations (example of Steam Assisted Gravity Drainage) is demonstrated, emphasizing that thermal oil production methods change reservoir rock behavior. Under elevated temperature and pressure, large changes in porosity, permeability, and compressibility occur; hence, the reservoir response evolves in time, a factor generally ignored in simulation. Whereas these geomechanical changes are largely beneficial as they tend to accelerate recovery rates, some difficult operational issues may arise, including casing shear, breach of reservoir seal, and excessive heat loss.