π(x) counts primes ≤ x. Gauss's conjecture: π(x) ≈ x/ln x. Riemann's improvement: π(x) ≈ Li(x) = ∫₂ˣ dt/ln t. Li(x) always overestimates — until the Skewes number (~10^316), where π(x) first exceeds Li(x).