This paper describes a combined observational and theoretical study of a lake breeze over the southern shore of Lake Ontario on 3 October 1972. The structure of the lake breeze circulation and its variations in time were obtained from observations. The observational data were gathered with a specially designed array of surface stations, tethered balloons, pilot balloons, aircraft and buoys. A two-dimensional, time dependent, primitive equation model was constructed to simulate the observed lake breeze. The numerical simulation is reasonably realistic; however, the lake breeze intensity and Wand penetration were under-estimated.