Velocity Prediction Programs (VPPs) based on a steady-state equilibrium between aero- and hydrodynamic forces continue to be important tools when assessing the performance of yachts during the design process. Over the last decade a number of Dynamic Velocity Prediction Programs (DVPPs), which also allow study of the dynamic characteristics of the boat, have been developed. Most DVPPs are based on numerically solving the equations of motion of the yacht according to Newton's second law with the aerodynamic forces being calculated from quasi-steady theory. This paper discusses whether this assumption of quasi-steady aerodynamics can be justified and also analyses the error introduced by such a quasi-steady analysis. Unsteady potential flow theory is used to predict the pressure distribution on an aerofoil-like, two-dimensional "slice" of a mainsail carrying out harmonic oscillations both perpendicular to, and along the direction of the incident flow. Such types of motion occur when a yacht pitches or rolls in waves. Theoretical pressure distributions are compared to wind tunnel measurements on an oscillating, rigid mainsail model of 3.2 metre span and 0.447 metre chord length. Experiments were carried out at reduced frequencies ranging from k = 0 to k = 0.8, as the mainsail of an International America's Cup Class yacht sailing upwind in waves typically encounters reduced frequencies in this range. It is found that predictions based on unsteady theory match the measured pressure distributions much better then quasi-steady predictions. This leads to the conclusion that, if the performance of the yacht is to be predicted on a time-scale shorter then the pitching period, this can be achieved best with an unsteady aerodynamic model. In the paper no attempt is made to investigate the influence of the flexibility of the sails, sail interaction, three-dimensional effects or phenomena related to dynamic stall.