This paper deals with the modular realization of sequential machines using shift registers as the modules. For a given minimal machine, the minimum number of modules required for its realization has been determined. It is well known that the number of modules required to realize a given machine can sometimes be reduced by realizing a non-minimal machine equivalent to the given minimal one, i.e. by assigning multiple codes to the states of the given minimal machine. This case is dealt with in the second part of the paper.