Pore- and fracture-pressure determinations are key considerations for successful well planning and drilling. Their accuracy has a large impact on the cost of a project and is especially critical for drilling high-temperature/high-pressure (HTHP) wells where there is a small margin between pore and fracture pressures. This paper outlines the method for calculating pore and fracture pressures using the effective-stress approach. This allows us to model downhole pressures and stresses using physical rock-mechanics laws, removing many of the uncertainties regarding traditional methods. Accurate pore-pressure, effective-stress, and fracture-pressure values are also important inputs for related technologies such as wellbore stability, basin modeling, sand production, reservoir subsidence and collapse.