Islanding operation is a special operation mode in the smart grid.For a power transmission system，if a connected system is split into several stable islanding sub-systems in time in the optimal islanding operation mode in case of extreme emergency，not only will enormous losses due to large area blackout or even system collapse caused by the spread of the local fault be avoided，but the amount of load shedding and generator tripping might be minimized as well.For a distributed system with distributed generator（DG），optimum islanding operation can bring the potential of DG into full play，while improving the reliability of power supply.The influencing factors of the optimal island partitioning in the smart grid are summed up and all the relevant methods and strategies available at home and abroad are thoroughly analyzed and classified.And the advantages and disadvantages along with key issues of each method are also discussed.Finally，the development tendency and the problems demanding prompt solution are treated.