While most propulsion systems which drive aquatic and aerial vehicles today are based on rotating blades or foils, there has recently been renewed interest in the use of oscillating foils for this purpose, similar to the fins or wings of biological swimmers and flyers. These propulsion systems offer the potential to achieve a much higher degree of manoeuvrability than what is possible with current man-made propulsion systems. There has been extensive research both on the theoretical aspects of oscillating-foil propulsion and the implementation of oscillating foils in practical vehicles, but the current understanding of the physics of oscillating foils is incomplete. In particular, questions remain about the selection of the appropriate structural properties for the use of flexible oscillating foils which, under suitable conditions, have been demonstrated to achieve better propulsive performance than rigid foils. This thesis investigates the effect of the foil inertia, stiffness, resonant frequency and oscillation kinematics on the thrust generation and efficiency of a flexible oscillating-foil propulsion system. The study is based on experimental measurements made by recording the applied forces while driving foil models submerged in a water tunnel in an oscillating motion using servo-motors. The design of the models allowed for the construction of foils with various levels of stiffness and inertia. High-speed photography was also used to observe the dynamic deformation of the flexible foils. The results show that the frequency ratio, or ratio of oscillation frequency to resonant frequency, is one of the main parameters which determines the propulsive efficiency since the phase of the deformation and overall amplitude of the motion of the bending foil depend on this ratio. When comparing foils of equivalent resonant frequency, heavier and stiffer foils were found to achieve greater thrust production than lighter and more flexible foils but the efficiency of each design was comparable. Through the development of a semi-empirical model of the foil structure, it was shown that the heavier foils have a lower damping ratio which allows for greater amplification of the input motion by the foil deformation. It is expected that the greater motion amplitude in turn leads to the improved propulsive performance. Changing the Reynolds number of the flow over the foils was found to have little effect on the relation between structural properties and propulsive performance. Conversely, increasing the amplitude of the driven oscillating motion was found to reduce the differences in performance between the various structural designs and also caused the peak efficiency to be achieved at lower frequency ratios. The semi-empirical model predicted a corresponding shift in the frequency ratio which results in the maximum amplification of the input motion and also predicted more rapid development of a phase lag between the deformation and the actuating motion at low frequency ratios. The shift in the location of the peak efficiency was attributed to these changes in the structural dynamics. When considering the form of the oscillating motion, foils driven in combined active rotation and translation motions were found to achieve greater efficiency but lower thrust production than foils which were driven in translation only. The peak efficiencies achieved by the different structural designs relative to each other also changed considerably when comparing the results of the combined motion trials to the translation-only cases. To complete the discussion of the results, the implications of all of these findings for the design of practical propulsion systems are examined. / Graduate / 0548