Since its inception, data assimilation has proved to be enormously useful in the most varied fields throughout the Earth sciences. It is certainly essential in meteorology, where the short-range forecasts would otherwise be almost impossible. In oceanography, its development has been slower, partly due to the smaller number of continuous and stable observations and partly due to fewer studies that show the importance of ocean forecasts for societal benefit. However, recently, these techniques have been used more and more widely, both in operational oceanography and to produce climate reconstructions. Although the techniques are similar to those used in the atmospheric field, they have to deal with particularities due to the different environment, where the boundary conditions, open and closed, have greater importance and the sparsity of observations pose unique challenges. This special issue is open to contributions describing data assimilation techniques, both methodological and case studies, in the oceanographic field. We welcome original work describing new techniques or new types of observations that cover every aspect of data assimilation, including varied applications of data assimilation, both in coastal seas and in the open ocean. This issue was inspired by the session OS4.7 at EGU2022.