This paper presents experimental and theoretical studies on the propulsive efficiency of oscillating foils. The research focuses on the flow characteristics around and in the wake of a high-aspect-ratio foil, using force and power measurements, as well as visualization data. The study shows that high propulsive efficiency, up to 87%, can be achieved under optimal wake formation conditions. Visualization results reveal that high efficiency is associated with the formation of moderately strong leading-edge vortices on alternating sides of the foil, which convect downstream and interact with trailing-edge vorticity to form a reverse Kármán street. The phase angle between transverse oscillation and angular motion is a critical parameter affecting the interaction of leading-edge and trailing-edge vorticity, as well as the efficiency of propulsion. The study compares experimental results with theoretical predictions of linear and nonlinear inviscid theory. It is found that agreement between theory and experiment is good over a certain parametric range when the wake consists of an array of alternating vortices and either very weak or no leading-edge vortices form. The results show that the optimal conditions for high efficiency are achieved when the heave amplitude-to-chord ratio is at its highest tested value, the phase angle between heave and pitch is around 75 degrees, and the maximum angle of attack is in the range of 15 to 25 degrees. The highest recorded efficiency in these experiments for high thrust production was equal to 87%, achieved under these optimal conditions. The study also compares the experimental results with numerical simulations and finds that nonlinear theory is somewhat closer to experiment than linear theory for higher values of the Strouhal number, but it is still in disagreement with experiment. The disagreement is more pronounced when small angles of attack are combined with small amplitudes of motion. The results suggest that the formation and manipulation of leading-edge vortices are key factors in achieving high efficiency. The study concludes that high efficiency is obtained for large heave motions and large angles of attack. The results also show that the delay of stall effects to large maximum angles of attack, due to unsteady flow effects, is in agreement with previous findings. The study highlights the importance of the phase angle between heave and pitch motion in determining the efficiency of propulsion.This paper presents experimental and theoretical studies on the propulsive efficiency of oscillating foils. The research focuses on the flow characteristics around and in the wake of a high-aspect-ratio foil, using force and power measurements, as well as visualization data. The study shows that high propulsive efficiency, up to 87%, can be achieved under optimal wake formation conditions. Visualization results reveal that high efficiency is associated with the formation of moderately strong leading-edge vortices on alternating sides of the foil, which convect downstream and interact with trailing-edge vorticity to form a reverse Kármán street. The phase angle between transverse oscillation and angular motion is a critical parameter affecting the interaction of leading-edge and trailing-edge vorticity, as well as the efficiency of propulsion. The study compares experimental results with theoretical predictions of linear and nonlinear inviscid theory. It is found that agreement between theory and experiment is good over a certain parametric range when the wake consists of an array of alternating vortices and either very weak or no leading-edge vortices form. The results show that the optimal conditions for high efficiency are achieved when the heave amplitude-to-chord ratio is at its highest tested value, the phase angle between heave and pitch is around 75 degrees, and the maximum angle of attack is in the range of 15 to 25 degrees. The highest recorded efficiency in these experiments for high thrust production was equal to 87%, achieved under these optimal conditions. The study also compares the experimental results with numerical simulations and finds that nonlinear theory is somewhat closer to experiment than linear theory for higher values of the Strouhal number, but it is still in disagreement with experiment. The disagreement is more pronounced when small angles of attack are combined with small amplitudes of motion. The results suggest that the formation and manipulation of leading-edge vortices are key factors in achieving high efficiency. The study concludes that high efficiency is obtained for large heave motions and large angles of attack. The results also show that the delay of stall effects to large maximum angles of attack, due to unsteady flow effects, is in agreement with previous findings. The study highlights the importance of the phase angle between heave and pitch motion in determining the efficiency of propulsion.