Gravity currents are flows that occur when a horizontal density difference causes fluid to move under the action of gravity; density currents are a particular case, for which the scalar causing the density difference is conserved. Flows with a strong effect of the horizontal density difference, even if only partially driven by it--such as the current of a sediment laden river discharging into a lake--are also considered gravity flows. The front of the current is a very small region of it, but in many cases determines the conditions in which the flow evolves. The front is rich in structures of different scales, arisen from shear and baroclinic instabilities. A defining characteristic of the front is the intense mixing that occurs at its interface. This work presents experimental results on density current fronts. The front of a density current is highly variable. Particle Image Velocimetry provides the spatial and temporal resolution to examine the velocity field; Laser Induced Fluorescence does the same for the density field. Simultaneous measurements of these two techniques--that resulted in the development of novel measuring and processing methodology--are extensively used to characterize the flow, as they show in detail the evolution of instabilities of the front interface, and in general of the mixing processes. The laboratory facility used allows the study of the front in a stationary setting, and also in its natural advancing state, for the case of a constant input of dense fluid to the system. These two settings are complementary, as the former highlights the mixing processes at the interface of the front, while for the latter the interaction between the current and the channel bed can be examined. Results from these experiments are presented, which characterize the flow in terms of the front velocity, the dimensions of the front, the mixing rate and the bed friction coefficient; the variability of these two last quantities is examined in detail. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]