This book presents the most recent achievements in data assimilation in Geosciences, especially in regards to meteorology, oceanography and hydrology. It spans both theoretical and applied aspects with various methodologies including variational, Kalman filter, maximum likelihood ensemble filter and other ensemble methods. Besides data assimilation, other important topics are also covered including targeting observation, parameter estimation, and remote sensing data retrieval. The book will be useful to individual researchers as well as graduate students as a reference in the field of data assimilation.