A theory of rotating spiral waves in excitable media is presented that allows the study of dynamical aspects such as nonsteady rotation. An approximate spiral-wave solution is proposed in the form of a superposition of curved solitary wavefronts, parallel to each other. This form is used, by means of singular perturbation theory, to derive an evolution equation for the spiral arm. Numerical solutions of that equation are found that describe one- and two-frequency rotations. The transition to two-frequency dynamics (tip "meandering") is attributed to a destabilizing curvature effect.