In the process of metal machining, the state of the cutting tool has great influence on the efficiency of production and surface quality, therefore, tool wear online monitoring is of great significance. The commonly used online tool wear monitoring methods in recent years were introduced, including cutting forces, vibrations, acoustic emission, temperature, current and power signal. The monitoring methods of each signal and its characteristics were analyzed and compared with the experimental results. It is proposed that multi-sensor synthesis technique can effectively avoid the shortcoming of the method of using one single test.