The entrainment of gravity currents advancing over a horizontal bed was studied. A two-dimensional rigid-lid flow model was derived assuming ambient-fluid entrainment to the mixing region being supplied only from the bottom layer of the approaching flow. Two sets of laboratory experiments were carried out using the laser-induced fluorescence (LIF) flow visualization technique. With given parameters such as the total fluid depth, densities of the fluids, height of the gravity current head and its propagation speed, and the denser-fluid flow depth behind the head under the mixing region, our model predicts that the thickness of the front flow layer to be entrained is about 35 percents of the height of the gravity current head. Qualitative examination of the flow structures along various planes in the developed fronts suggests that the actual flow structures at the foremost part of the current head are complex and three dimensional. Entrainment of ambient fluid to the current is through various directions starting at its front, which creates an unstable stratification condition there favorable for the subsequent complex three-dimensional eddy generation and growth leading to the formation of the short-crested billows exhibiting the lobe-and-cleft features in the following flow. Copyright © 2011 by ASME.