Based on the introduction of some concepts about hydrological scale, scale issue and scaling, four approaches to scale were discussed,namely, fractal theory,-wavelet principle, distributed modeling, and chaotic theory. The key problems were put forward related to the scale issue in hydrology such as uncertainty principle in the scale problem, observing scale and hydrological processes of different scale-up, and the spatial heterogeneity of hydrologic phenomenon. Existing problems and prospects of research on hydrological scale were put forward. It was pointed out that the combination of various techniques and theories would motivate hydrologic scaling research and the research of hydrology scale would drive the development of hydrological science.