Spiral waves arise in excitable media — cardiac tissue, the BZ reaction, neural tissue. The FitzHugh-Nagumo model: ∂u/∂t = D∇²u + u(u-a)(1-u) - v, ∂v/∂t = ε(u - βv). The activator u spreads, while inhibitor v recovers. Broken wavefronts curl into stable rotating spirals — the origin of cardiac arrhythmia.