The ratio of the predicted critical heat flux (CHF) to the reactor heat flux is used as an index of the power capability of water reactors. Numerous boiling heat-transfer and two-phase-flow studies over the past 15 years have put emphasis on exploring the boiling-crisis mechanisms and on perfecting the CHF predictions. Unfortunately, the problem is more complicated than was thought. Thus far no overall analytical solution has been obtained, and the design equations for CHF predictions arc empirical. In this review various types of boiling crisis are categorized according to four major flow patterns: pool boiling, bubbly flow, annular flow, and normal flow across a cylinder. The physical mechanisms of these boiling crises are described. Experimental observations are summarized and used for verifying various analyses. The missing links in the analyses are identified, and future work is recommended.