A data assimilation strategy based on feedback control has been developed for the geophysical sciences —a strategy that uses model output to control the behavior of the dynamical system. Whereas optimal tracking through feedback control had its early history in application to vehicle trajectories in space science, the methodology has been adapted to geophysical dynamics by forcing the model's trajectory to follow observations in accord with observation accuracy. Fundamentally, the approach is based on Pontryagin's minimum principle where a least squares fit of idealized path to dynamical law follows from Hamiltonian mechanics. This utilitarian process optimally determines a forcing function that is appended to the dynamical law. Nudging, a data assimilation process that has enjoyed wide appeal in meteorology, is a special case of this method. Following development of the theory for both linear and nonlinear systems, application is made to a simple air/sea interaction model.