This paper discuss the theory of orientation estimation algorithms designed for inertial/magnetic sensor modules and briefly describe three types of sensor modules. Specifically, the modules discussed are the InterSense InertiaCube, the MicroStrain 3DM-G, and the MARG III. The MARG III was designed by the authors and manufactured by McKinney Technology. Basic background on the ambient magnetic field of the Earth and how it is distorted by ferrous objects and electrically powered devices is then provided. Methods of calibrating magnetic field variations are then discussed.