Recently, technological advances have contributed to a significant increase inthe use and scale of directional drilling. Perhaps the technologies with thehighest impact have been steerable mud motors, measurement-while-drilling(MWD) tools, and logging-while-drilling (LWD) tools. These tools incombination have provided the ability to follow complex, 3D well profileswithout changing bottom hole assemblies (BHAs), and to measure where the bit has drilled without having to run a wireline to survey or log. Equallyimportant, engineering models have provided the fundamental tools forevaluating drill strings, hydraulics, BHAs, and the drilled formations themselves.