Flows of granular materials widely occur in many industrial and natural environments. Particle-size segregation---the separation of grains of different sizes during flow---plays a crucial role in the behaviour of such flows. In spite of considerable efforts over the last years to investigate size-segregation, the phenomenon is still only partially understood. In particular for dense gravity-driven flows, where small particles percolate to the bottom of the flow and large particles rise to the top, against gravity, the particle-scale behavior and mechanisms are poorly understood. This thesis presents an experimental and particle simulation investigation of size-segregation in dense gravity-driven bi-disperse granular flows in various three-dimensional configurations. Namely, an experimental shear-box, a numerical chute flow, an experimental moving-bed channel, and a numerical moving-bed channel. Using the non-intrusive imaging technique Refractive Index Matched Scanning (RIMS) we investigate the particle motion inside the bulk of the experimental configurations. The moving-bed channel was designed from the ground up with the use of the RIMS technique in mind. The results obtained through both experiments and simulations demonstrate the existence of a property we call `size-segregation asymmetry'. This property is fundamental to the process of size-segregation and explains a number of specific features that arise on the bulk-scale of the flow. It can be simply and intuitively explained as follows: When a large particle in a granular flow is surrounded by many small particles it rises to the free-surface at a low velocity compared to the sinking velocity of a small particle in a flow surrounded by many large particles. We quantify this behaviour by measuring the segregation velocities as a function of local particle concentration. Comparison of the bulk concentration field with predictions of an asymmetric size-segregation model allow us to link the particle-scale behaviour to a number of bulk-scale features. Segregation at the front of granular avalanches produces a recirculation zone, known as a `breaking size-segregation wave', in which large particles are initially segregated upwards, sheared towards the front of the flow, and overrun before being resegregated again. This recirculation zone separates a coarse particle front from a small-particle tail. By making use of a moving-bed channel, which permits the creation of a continuous gravity-driven flow, we are able to visualise, using RIMS, the complex internal structure of breaking size-segregation waves for the first time. We find that size-segregation asymmetry plays a role in the observed structure. In particular, it is seen that a few large particles are swept a long way upstream inside the small-particle tail and take a very long time to recirculate. Additionally, a basal slip is observed that is linked to the local presence of the different sized particles, with the tail of the flow experiencing less friction compared to the front. By studying the basal slip we are able to explain the complex relation between the mixture ratio of the two species and the flow speed. Overall, this thesis highlights the important role of the distinct dynamics of large and small particles in size-segregating granular flows.