Gravity currents are created by a continuous release of dense fluid along a 6¿ inclined ramp. The surrounding fluid is composed of a two-layer stably stratified environment. A chemical (phenolphthalein) reaction technique and Planar Laser Induced Fluorescence (PLIF) are used to measure the entrainment rate of lower fluid into the gravity current. The change of volume in the head of the gravity current quantifies the entrainment and mixing. Their dependence on the Richardson number, based on the gravity current head size and velocity and the initial density difference between the current and bottom layer fluids, is determined using chemically reacting PLIF. To further understand the entrainment process and to quantify the effect of the interface on the internal structure of the gravity current, Particle Image Velocimetry (PIV) measurements are performed. Both instantaneous velocity and vorticity fields are quantified. In addition, vertical profiles of velocity and vorticity are computed at two different locations, upstream and downstream of the impingement region. The vortical structure of the gravity current is clearly affected by the presence of the stratified interface. The level of restructuring depends on the Richardson number, based on the density difference across the interface. At low Ri, vorticity increases following the impingement, whereas at higher Ri, a decrease in vorticity is observed as well as a significant change in the head structure of the gravity current.