This thesis is concerned with the measurement of self-aerated flow on the spillway of a large dam. The major part of the project involved the development of an air concentration probe and a velocity meter suitable for use under field conditions. The air concentration probe basically measures the electrical conductivity of a small filament of the flowing air-water mixture. The velocity meter measures the stagnation pressure at a point in the flow and relates this to the velocity and air concentration. Results subsequently obtained with the instruments on the spillway of the Aviemore Dam are presented. During testing, it was noted that the “critical point”, where aeration commenced, moved downstream with increasing flow discharge. Furthermore, it was found that its position could be predicted by means of a formula derived from a previous experimental study of model overflow spillways. An analysis of model data, obtained in a previous model study, was carried out and found to correlate well with the prototype data obtained on the Aviemore spillway. A design method for partially - aerated developing flows is presented. It was found that the governing criterion in spillway design is the extent of wall-induced aeration. Further development of the instruments to enable measurements to be made near the spillway side wall is discussed.