This study is directed towards developing a better understanding of the erosion mechanisms involved in the use of high-pressure water jets for cutting in rock materials. Theories to explain the cutting action of high-pressure water jets have been proposed by Crow (Int. J. Rock Mech. Min. Sci. & Geomech. Abstr. 10, 567-584, 1973 and 12, 203-212, 1975) and Rehbinder  (Rock Mech. 12, 247-257, 1980). These theories predict the depth of kerf cut by the jet in terms of the rock and fluid properties and the water jet system parameters. While both theories produce reasonable results for certain cases, neither is completely satisfying. For pedagogical purposes this paper is organized in sections. Following the Introduction, a review is given of published empirical findings from studies of rock erosion using high-pressure water jets. Then the theories of Crow and Rehbinder are compared with published experimental results to determine how well these two models predict the kerf depth in rock cut by a water jet. Next, results are given of new water jet rock kerfing experiments conducted using a factorial approach in the experimental design. Empirical models fitted to these results identify the relative importance of both the rock properties and the water jet parameters in the erosion process. There follows a description of a new laboratory technique employed to examine micro-structural rock damage caused by the water jet in eroding the rock. The objective of all of this work is to gain new insight into the mechanisms of rock erosion. The concluding section then outlines a proposed new physical model for rock erosion by a high-pressure water jet.