The importance of surfare condition on nucleate boiling has long been recognized. It has also been known that only cavities of a narrow size range can be active nucleation sites. In order to define the size range of active cavities as a function of wall temperature or heat flux, a model is proposed. The model pictures a bubble nucleus at a site enveloped by a warm liquid. The nucleus will begin to grow into a bubble only when the surrounding liquid is sufficiently superheated. The time required for the liquid to attain this superheat is called the waiting period. The transfer of heat from the superheated liquid into the bubble is considered to be a transient conduction process. A cavity is considered effective only if the waiting period is finite. This criterion gives the limiting sizes of effective cavities. The equations show that maximum and minimum sizes of effective cavities are functions of subcooling, pressure of the system, physical properties, and the thickness of the superheated liquid layer. Comparison of theoretical prediction with experimental data from several sources was made. The fluids considered were ether, pentane, and water, with water under various degrees of subcooling. The theory did predict the incipience of boiling and size range of cavities successfully.