The Brusselator: du/dt = A − (B+1)u + u²v + D_u∇²u, dv/dt = Bu − u²v + D_v∇²v. Turing instability occurs when the homogeneous steady state (u*=A, v*=B/A) is stable without diffusion but destabilized by diffusion. The condition D_u/D_v < D_u*/D_v* with B > 1+A² triggers stripe or spot patterns.