A review of important issues in steam injection in naturally fractured reservoirs (NFRs) is presented. The effect of temperature on physical properties of crude oils and rocks and the thermochemical alteration of crude oil are discussed. The recovery of oil from NFRs can be modelled as a two step process: first the oil is expelled from the matrix blocks through mechanisms that impose a pressure gradient within each matrix block and then it is swept through the fracture network to a production well by mechanisms that impose a pressure gradient within the fracture network. The recovery mechanisms associated with steam injection in NFRs and their characteristic times are presented. The most important recovery mechanism in matrix blocks is differential thermal expansion between oil and the matrix pore volume and the strongest mechanism in fracture network is the reduction of viscosity ratio (mu(o)/mu(w).). The matrix oil recovery mechanisms are relatively independent of oil gravity, making steam ail equally attractive recovery process in fractured light and heavy oil reservoirs. The mechanism and impact of CO(2) generation during steam injection in carbonate reservoirs arc discussed. The rate of CO(2) generation is controlled by the rate of heat conduction from fracture into the matrix. For a specific reservoir the rate of heat conduction is a function of temperature and injection rate of steam and these call be optimized to make use of the in situ generated CO(2).