Plate heat exchangers were first developed about 100 years ago, but have won increasing interest during the last two decades, primarily because of the development of methods of manufacturing brazed plate heat exchangers. This type of heat exchanger offers very good heat transfer performance in single-phase flow as well as in evaporation and condensation. Part of the reason is the small hydraulic diameters, typically being less than 5 mm. Other advantages of plate heat exchangers are the extremely compact design and the efficient use of the construction material. In spite of their long use, the calculation methods for predicting heat transfer and pressure drop are not widely known. It is the purpose of this article to present such calculation methods for singe phase flow and for flow boiling and to discuss some of the specifics of this type of heat exchangers.