The WKB (Wentzel-Kramers-Brillouin) approximation gives the semiclassical wavefunction ψ(x) ≈ A/√p(x) · exp(±i∫p dx/ℏ) where p(x) = √(2m(E-V(x))). In classically forbidden regions (E < V), momentum becomes imaginary and the wave decays exponentially. The tunneling transmission T ≈ exp(-2∫|κ|dx/ℏ) where κ = √(2m(V-E))/ℏ — visible here as the exponential decay through the barrier. The Bohr-Sommerfeld quantization condition ∮p dx = (n+½)·2πℏ emerges from matching WKB solutions at turning points via connection formulas.