A two part experimental study of gravity currents flowing in horizontal channels is performed. The first of these examines adiabatic aqueous flows, and the second, heat transferring gaseous currents. Video photography of dyed saline current fronts flowing into a fresh-water-filled duct allows for front velocity measurements and the study of qualitative flow features such as current head structure, mixing, and layer thickness. In conjunction with previous works, a model of gravity current entrainment is presented. The effects of viscosity are examined in the transition from constant velocity flow to a decelerating, viscous dominated flow regime. This transition is shown to occur at a streamwise position that is a function only of the current layer thickness, and not of Reynolds number, as previously believed. To examine the effects which the reduction of buoyant forces due to heat loss has on gravity current flow, heated gaseous flows in another experimental facility are studied. A smoke flow-visualization technique is developed, and together with fine gauge thermocouples, heat flux gauges, and shadowgraphy, provides for flow measurement. Front velocities, layer thicknesses, wall heat fluxes, layer velocity and temperature profiles, and qualitative flow characteristics are recorded. The thermal front, with its constant channel-ceiling-temperature boundary condition, is found to decelerate as it moves downstream, while the layer thickness is seen to increase. Layer Richardson number is found to be constant along the streamwise direction of the flow, and an empirical relation for Nusselt number versus Reynolds number is obtained. Free convection in the form of longitudinal roll cells is shown to promote the high levels of heat transfer measured. A flow modeling scheme based on experimental observations is also developed.