AbstractSteam-Assisted Gravity Drainage (SAGD) is the main commercial technology used for in-situ recovery of Canadian heavy oil and Bitumen. It is commercially proven and delivers high oil rates and high ultimate recoveries. One of the long-term concerns with the SAGD process is high energy intensity and related environmental impacts.Hybrid processes have been developed to take partial advantage of steam and solvent processes while introducing a more efficient and more economically viable recovery methods. Several processes such as Propane-SAGD, Expanding Solvent-SAGD (ES-SAGD), Solvent-Aided Process (SAP), Liquid Addition to Steam to Enhance Recovery (LASER) and Steam-Alternating-Solvent (SAS) were proposed; some of them currently under pilot test. Hybrid steam-solvent processes aim to accelerate oil production rate with lower cost than SAGD and also increase the ultimate oil recovery.Despite remarkable amount of laboratory and computational studies on these processes, there was no extensive critical review of the knowledge obtained for more than a decade. The current level of understanding of the hybrid processes and knowledge around the fundamental physics and mechanisms involved are not fully satisfactory. We believe that a critical review of the status of the hybrid processes will fill the gap by shedding the light on the deficiencies and the limitations of the process, further development areas, and new research topics. Analytical, numerical simulations, laboratory modeling efforts along with pilot test results are summarized. In addition, the main technical challenges of different aspects of hybrid steam-solvent processes are analyzed at different levels. In this paper, special attention is given to a) The effect of reservoir and operational parameters, b) solvent injection strategies, c) The inconsistency between laboratory, simulation and field results and d) problems faced in numerical modeling (capturing the physics of heat and mass transfer).It is believed that a good compilation of the records produced over one decade will constitute a useful reference for the industry and academics. Analytical, simulation, laboratory studies and reported field data strongly support hybrid steam-solvent processes. However, the results are mixed at different level levels and there exists some inconsistencies. The cost of the solvent retained in the reservoir is the major concern and the economics of selected hybrid steam-solvent process for a specific reservoir has to be verified using available tools. The main challenges are verifying effective mixing of the solvent with the in-situ bitumen, managing the solvent placement and distribution in the reservoir, reliably determining the incremental benefit of solvent-addition and ensuring economic solvent recovery.