The determination of in-situ stresses is important in Enhanced Geothermal System (EGS) energy development. Large-scale stress changes take place during EGS, affecting rock mass permeability, expecially in naturally fractured rock masses, and potentially even triggering felt microseismic activity. Good predictive modeling is vital for decisions on project viability and environmental impact, therefore good stress data are vital. Hydraulic Fracturing (HF) is a widely accepted technique for in-situ stress determination, but is time-consuming and expensive because of rig time involved in lowering the tools to take measurements. In contrast, taking advantage of the shape of borehole breakouts measured from widely-available caliper and image logs to estimate in-situ stress in deep hot rocks is more economical, requires no extra rig time, and is thus an attractive alternative. Four-arm caliper data and image logs are standard borehole geophysical logs, and more sophisticated logs may be used if more data are required (such as in a research borehole or the first several boreholes in a multi-well EGS program).