In digital communication systems, the modulation classification has emerged with new concern. This is mostly considered necessary in Communication Intelligence (COMINT) applications such as signal interception for defence, civil authority, surveillance and threat analysis, etc. Recently many algorithms have been projected to distinguish digitally modulated signals. In this paper, we present and evaluate some of the major problems, approaches and algorithms to recognize automatically the type of the modulated signals. First of all, Azzouz and Nandi’s algorithm has been compared and discussed. To simplify our study, here some classical digital modulations schemes have been considered only.  Many simulations have been carried out and presented for these modulation types by using statistical approaches.