In this paper,we introduce several light path monitoring techniques in automatic switched optical networks(ASONs),including indirectly monitoring method,node-internal identification tag method and global unique identification method,such as pilot tone.The principles,the experimental results,the realizabilities,the advantages and disadvantages of these techniques are described and compared.The comparisons show that indirectly monitoring method is a practically adopted technique due to its simplicity;digital wrapper will be a preferred technique in industry in the near future for its advantage and realizability;other techniques are in laboratory research stage.