Flood inundation mapping is an important tool for municipal and urban growth planning, emergency action plans, flood insurance rates and ecological studies. Mapping floodplain is a crucial component in flood risk reduction measures. With the companion GIS utility especially Arc-GIS and HEC-GeoRAS, the present work performs a flood inundation mapping study using HEC-RAS and presented a case study on Gorakhpur City and its environ. Gorakhpur city and its peri-urban areas are infamous for frequent flood hazard. Being situated on the floodplain of the lower Rapti river basin, it comes under the flat middle Ganga plains, India. The study used a vector based GIS and DEM in order to delineate watershed boundaries and predict areas of possible inundation during a flood event in and around the city of Gorakhpur. Steady flow analysis was performed and flood extent was then simulated based on those derived parameters and different flood peak scenarios were generated using 2, 25, 50 and 100 year return period. The book is useful for scholars working in the field of hydrology, urban planning, geography, and GIS.