Alignment is the process whereby the orientation of the axes of an inertial navigation system is determined with respect to the reference axis system. The basic concept of aligning an inertial navigation system is quite simple and straight forward. However, there are many complications that make alignment both time consuming and complex. Accurate alignment is crucial, however, if precision navigation is to be achieved over long periods of time without any form of aiding. In addition to the determination of initial attitude, it is necessary to initialise the velocity and position defined by the navigation system as part of the alignment process. However, since it is the angular alignment which frequently poses the major difficulty, this chapter is devoted largely to this aspect of the alignment process.