  We make a short review about the synchronization in coupled phase oscillator
models. Next, we study the common-noise-induced synchronization among active
rotators. At an intermediate noise strength, the noise-induced synchronization
takes place most effectively, which is analogous to the stochastic resonance.
Finally, we study the synchronization of coupled phase oscillators with
nonvariational interaction on scale-free networks. We find a sharp transition
and a weak hysteresis in the nonvariational systems. The sharp transition is
found also in the mean-field approximation.
