Water flooding is the most widely applied recovery process in both conventional and heavy oil reservoirs. It is generally accepted that theories explaining water flooding performance in light oil reservoirs are not applicable to heavy oil reservoirs. Nonetheless, there is a lack of a systematic study discussing the underlying mechanisms of water flooding in heavy oil systems. This article presents findings of water flooding experiments and discusses the interplay between viscous and capillary forces as a function of oil viscosity and injection velocity. Experimental data of 178 core floods were used to develop a new dimensionless capillary number, , which is useful in predicting water flood performance for a wide range of oil to water viscosity ratio up to 15,000. This scaling parameter along with the Peters and Flock’s (Peters and Flock, 1981) instability number were used to map the interplay between capillary and viscous forces. It was found that there is a critical oil to water viscosity ratio (≤20) below which the flow is stable (having instability numbers below a critical value) and viscous-dominant. In these cases, breakthrough oil recovery monotonically increases with increasing injection velocity. For viscous oils with viscosity of greater than 160 mPa s, flow is identified as pseudo-stable flow with instability numbers above a critical instability number. In these cases, breakthrough oil recovery is almost independent of injection velocity. In intermediate oil to water viscosity ratio of 20–160, breakthrough oil recovery increases with decreasing injection velocity, suggesting the flow regime is capillary-dominant. In these cases, imbibition activated at slower velocities has been identified as the main mechanism responsible for incremental oil recovery. In viscous oil systems (160 < μo < 15,000), late time oil recovery monotonically increases with decreasing injection velocity. This increase in oil recovery is more pronounced in more viscous oil systems suggesting the importance of capillary forces in these systems. Results of this study suggest some new insights on the mechanisms of water flooding as a cost effective non-thermal EOR technique and how this can be very different in light oil systems compared to heavy oil reservoirs.