Viterbi decoders are used in wide variety of communication applications. In this paper, the authors focus on different types of VHDL implementations of Viterbi decoder. The two approaches of Implementation of Viterbi decoder are register-exchange approach and trace back approach. There are two methods in trace back approach i.e. shift update and selective update. The behavior of a Viterbi decoder is described in VHDL. A gate level circuit was obtained from the behavioral description through logic synthesis.