The disintegration of a plane sheet of liquid of finite thickness, moving tangentially relative to a surrounding fluid, is subjected to mathematical and experimental analysis. Instability and wave formation at the interface are established as the major factors in the breakup of the sheet of liquid into drops. An equation is derived relating four significant dimensionless groups, and a graphical presentation of the quantitative relationship among these groups is included. The force balance between the interfacial tension and aerodynamic forces is the basic concept, and the criteria for stability and instability are determined from it. It is possible to predict the conditions for maximum instability, and therefore for the most rapid disintegration of the sheet. Short-exposure photographs of the actual disintegration of the sheet are included to support the analysis, and a prediction of drop size in the spray compares favorably with measurements by other methods.