When a less viscous fluid (μ₁) displaces a more viscous one (μ₂) in a Hele-Shaw cell, pressure satisfies Laplace's equation. The flat interface is Saffman-Taylor unstable — perturbations grow by diffusion-limited aggregation (DLA). Surface tension regularizes the instability, selecting a single finger of width λ/2 at high velocity. The fractal dimension of DLA clusters: D_f ≈ 1.71.