Watershed transform is a classical method of image segmentation in mathematical morphology, and its parallelism is an important problem. This paper starts with a mathematical model of watershed transform in topography, followed by its several formal definitions for digital case. Then some new serial watershed algorithms proposed in recent years are classified and analyzed, based on which their related parallel strategies are discussed in detail. Then we analyze some issues, which should be considered when designing parallel watershed algorithm, and give an evaluation of existing parallel algorithms, and draw some conclusions. Finally we point out the problems and challenges of future research.