This chapter deals with hydrological data and analysis. Hydrological data collection and analysis for the proposed project site are required to work out the water availability for power generation, flood discharge for design of diversion structure and the diversion discharge required to plan the diversion scheme for the construction of diversion structure. This chapter contains the methods of collection and analysis of rainfall and run off data. At most of small hydro project site generally discharge observations are not available and so data of other sites are used and statistical procedures are used to convert them for the proposed site. The methods which are used with different sets of data are briefly explained. The methods of flood frequency and unit hydrograph are also described to work out the flood discharge. Development of stage discharge curve and estimation of sediment quantity are also briefly explained.