Critical heat flux (CHF) during pool boiling was discussed in Chapter 3. Similar phenomena also occur during forced convection boiling. A sharp deterioration in heat transfer occurs at some point with increasing heat flux or quality. The heat flux at which the deterioration in heat transfer occurs is known as the CHF and the corresponding quality is known as the critical quality. The mechanisms and prediction of CHF for various conditions and types of heat exchangers are discussed in this chapter. Included are flow inside channels such as tubes and annuli, coils, single tubes, tube bundles, plates, coils, falling films, jets, sprays, etc.