A theory of spiral waves is presented that includes nonlocal effects due to wave-front interactions. Evolution equations for the spiral wave front are derived from the basic reaction-diffusion system, using a singular perturbation method. It is shown that nonlocal effects play a crucial role in stabilizing the dynamics of spiral waves and may substantially affect their spatiotemporal behavior. In particular, conditions are found under which spiral cores expand in time. An expression for the normal velocity is derived and compared with previous results.