A study concerned with the determination of the limiting hydrodynamic conditions which characterize nucleate and transitional boiling was conducted. The critical heat flux and the minimum heat flux are derived from these hydrodynamic limits. Results of investigations of nucleate boilin g are discussed and the theory of bubble growth is extended to include the effect of non-uniform temperature fields. An equation is derived for the product bubble diameter times frequency of bubble emission, and data showing that the diameter of nucleating cavities can he related to the heat flux density and superheat difference are presented. (J.R.D.)