The book explains the methodology of soil moisture modelling and assimilation. The book begins with the modelling of soil moisture using the numerical solution of physical based equations. The book explains how can the measurements be used in improving the state of the model, hence in improving the predictions by means of data assimilation. The linear Kalman, Extended Kalman and Ensemble Kalman filtering approaches are given for data assimilation. The approaches to measure the soil moisture in field, and the way to analyse the field data by means of the variogram analysis are also explained. The parameter estimation using inverse modelling is also covered in the book.