We investigate the Navier-Stokes initial boundary value problem in the half-plane $R^2_+$ with initial data $u_0 in L^infty(R^2_+)cap J_0^2(R^2_+)$ or with non decaying initial data $u_0in L^infty(R^2_+) cap J_0^p(R^2_+), p > 2$ . We introduce a technique that allows to solve the two-dimesional problem, further, but not least, it can be also employed to obtain weak solutions, as regards the non decaying initial data, to the three-dimensional Navier-Stokes IBVP. This last result is the first of its kind.