The hydro-mechanical effects of high pressure fluid injections into jointed rock are considered mostly in the context of Hot Dry Rock (HDR) geothermal energy systems. In Part I, the mai n aspects ari sing from the HDR research at the "Camborne School of Mines (CSM) and Los Alamos Nat iana 1 Laboratory (LANL) projects are reviewed. Previous approaches to fluid-rock interacti ons at these projects and important observed phenomena are highlighted. Fundamental aspects of rock joint geometry, mechanical behaviour and flow regimes within jointed rock are also reviewed. These aspects are then related to possible conditions in HDR systems. The role of in situ stress conditions is of great significance in this study and is reviewed theoretically and in detail for both the CSM and LANL project sites. The revi ew incl udes a comprehensi ve seri es of measurements, by different techniques, organised and interpreted by the author at the CSM project. In Part II, model development, the emphasis is on intermediate fluid pressures which are too high for simple diffusion alone and too low for tensile hydraulic fracturing. The dominant mechanical activity is one of joint shear. Strike-slip shearing due to fluid injection is examined in two dimensions with the numerical model FRIP, which has been extended by the author. Similar behaviour is examined in three dimensions with an analytical model which is linked to microseismic observations. This model explains the observed phenomenon of downward shear growth. Joint distribution and mechanical properties, and their effect on fluid diffusivity, are examined and used in analytical models of fluid pressure pulse propagation, tracer transport, and rock stress increment transfer. All models are used to help interpret field data, mainly from the CSM project. The models are also of potential application to hydrocarbon reservoir stimulation, liquid waste disposal and leakage from high pressure water tunnels.