This paper reviews the status of research reported in the open literature on plate heat exchangers, operating in single-phase and two-phase adiabatic and flow boiling applications. The review has been made as apart of an ongoing study aimed at enhancing the efficiency of heat transfer in plate heat exchangers used as evaporators in small heat pumps. It constitutes the background for the report of the activities within the project, given in part two of the present article. In order to obtain a complete picture of the performance of a plate heat exchanger operating as an evaporator, it is as important to have knowledge of the single-phase characteristics as it is to have knowledge of the flow boiling characteristics. The review shows that single-phase flow is quite well understood, even if the influence of the many geometric parameters is still under discussion. It also shows that there is still a controversy concerning which mechanism is dominant in flow boiling: convective evaporation or nucleate boiling.