The phenomena governing the critical heat flux (CHF) in flow boiling in ducts and rod bundles vary with the flow pattern. Separate models are needed for dryout in annular flow, wall overheating in plug or slug flow, and formation of a vapor blanket in dispersed flow. The major theories and their current status are described for the annular and dispersed regions. The need for development of a theoretical approach in the plug and slug flow region is indicated. Current phenomenological models are capable of providing satisfactory predictions of rod bundle CHF in both the annular and dispersed flow areas. However, the precision obtained remains below that of empirical correlations.