Groundwater development has shown phenomenal progress in our country during past few decades. There has been a vast improvement in the perception, outlook and significance of groundwater resource. Groundwater is a dynamic system. It is dynamic in the sense that the state of any hydrological system is changing with time, and in the sense that we are continually developing new scientific techniques to evaluate these systems. The total annual replenishable groundwater resource of India is around 431 BCM. Inspite of the national scenario on the availability of groundwater being favourable, there are many areas in the country facing scarcity of water. This is because of the unplanned groundwater development resulting in fall of water levels, failure of wells, and salinity ingress in coastal areas. The development and over-exploitation of groundwater resources in certain parts of the country have raised the concern and need for judicious and scientific resource management and conservation. A complexity of factors - hydrogeological, hydrological and climatological, control the groundwater occurrence and movement. The precise assessment of recharge and discharge is rather difficult, as no techniques are currently available for their direct measurements. Hence, the methods employed for groundwater resource estimation are all indirect. Groundwater being a dynamic and replenishable resource is generally estimated based on the component of annual recharge, which could be subjected to development by means of suitable groundwater structures. Mathematical models are tools, which are frequently used in studying groundwater systems. In general, mathematical models are used to simulate (or to predict) the groundwater flow. Predictive simulations must be viewed as estimates, dependent upon the quality and uncertainty of the input data. Model conceptualization is the process in which data describing field conditions are assembled in a systematic way to describe groundwater flow processes at a site. The model conceptualization aids in determining the modelling approach and which model software to use. Taking the base from my lecture notes delivered in various training courses during last 26 years and further editing and additions, I have developed this book titled "Groundwater Assessment and Modelling". The book is intended to provide a comprehensive treatise related to assessment and modelling of groundwater. It includes chapters on assessment of groundwater potential, groundwater data requirement and analysis, basic concepts and guidelines for groundwater modelling, groundwater modelling software, modelling of unsaturated flow, modelling of sea water intrusion, and impact of climate change on groundwater resources. I hope this book will be quite useful for undergraduate and postgraduate students (water resources engineering), field engineers and researchers working in the area of assessment, development and management of groundwater resources. Any comments or suggestions to improve the contents of book are welcome.