External gear pumps make some of the most frequently used hydraulic energy generators. In spite of the fact that those well known machines feature good operational parameters, the research and development work on the pumps is still going on. There is a number of various research methods applied. One of them, an experimental method, is a visualization study, on which this paper is focused. In this article, the visualization research method has been presented, which has been used for the study of the flow processes and phenomena in the external gear pump. Findings of the research enabled a detailed description of the flow process, namely of the transporting of the working fluid through the pump, in each of the typical zones of the pump. It turns out that a characteristic feature of the process is the occurrence of various cavitation phenomena. All characteristic cavitation forms, as well as the degrees of their intensity, observed during the research, have been catalogued and specifically described. The research also made it possible to identify the critical areas of the pump, which, consequently, allowed the development of specific design, construction and operational recommendations. A result of applying those recommendations was a considerable reduction of the intensity of the cavitation phenomena, evidence of which is the experimental study presented in this paper.