We investigate multiuser synchronization problems fundamentally by using maximum likelihood estimation techniques. In particular, we put most efforts on multiuser carrier frequency offset estimation. Due to the special contour shape of ML estimation metric function, a two-stage estimation method including DFT coarse search and neighborhood search is described in single-user case and extended to multiuser case. We discuss multiple-access interference (MAI) in the presence of carrier frequency offset. From the maximum likelihood viewpoints, we suggest the successive interference cancellation and the parallel interference cancellation to improve near-far resistance. We also see the tracking capability of phase based CFO estimation method in multiuser environments.