Unsteady aerodynamics deals with air flows that can not be sufficiently described without considering the time dependence of the flow. Unsteady flow phenomena are encountered with wind turbines, helicopter rotors and aeroplane propellers. It is also an important factor in the study of animal flight, with birds, bats and insects flapping to produce lift and propulsion. Recently also micro aerial vehicles (MAV’s) have been constructed that use those same mechanisms for flight. With the availability of high-speed particle image velocimetry (PIV) systems, it is now possible to quantitatively measure unsteady flow fields in a time-resolved manner. Methods have been developed that enable to determine the unsteady aerodynamic forces on an object from the measured flow velocity field around it, using a control volume approach. For steady flows, these methods have been extensively validated previously by comparison to balance measurements. For unsteady flows, however, such a validation has not been made before. The present thesis investigation involves an experimental study of an aerofoil subjected to an oscillating pitch motion. Unsteady loads were determined from instantaneous velocity fields, that were obtained using a high-speed PIV system. Additionally it has been attempted to validate these loads by means of force balance measurements. The experimental set-up involved an actuated sinusoidally pitching aerofoil, with a NACA0018 profile. Eight strain-gauge bending-beam sensors formed a balance for the mechanical reference force measurements. With a PIV system, consisting of 4 high speed cameras, with partially overlapping fields-of-view, and a high speed laser, time-resolved velocity data was obtained in the cross-sectional plane of the aerofoil. Reynolds numbers near 80 000 and pitch frequencies of 10 and 20 Hz were investigated. The forces obtained from PIV agreed well with the balance data for the static cases. For the oscillating cases, they were comparable to those predicted using Theodorsen’s theory. The latter, a solution of the potential flow problem for a harmonically pitching and heaving flat plate, is an often used prediction method for unsteady aerofoil problems. The similarity between the PIV results and Theodorsen’s method, supports the credibility of the PIV results. The balance measurements were less successful for the aerofoil pitching at high frequencies. They suffered from the actuation forces and resonance with higher frequencies present in the aerofoil motion. This meant that many corrections had to be applied, which were based on estimations of the mechanical properties of the set-up.