We present some systematic approaches to the mathematical formulation and n
umerical approximation of the time-dependent optimal control problem of tra
cking the velocity for Navier Stokes flows in a bounded, two-dimensional do
main with boundary control. We study the existence of optimal solutions and
derive an optimality system from which optimal solutions may be determined
. We also de ne and analyze semidiscrete-in-time and full space-time discre
te approximations of the optimality system and a gradient method for the so
lution of the fully discrete system. The results of some computational expe
riments are provided.