After recombination (z ≈ 1100) the universe was filled with neutral hydrogen — the cosmic dark ages. Around z ≈ 6–12, the first stars and quasars emitted UV photons energetic enough to re-ionize hydrogen, carving out expanding HII bubbles (Strömgren spheres). The Epoch of Reionization ended around z ≈ 6 when bubbles percolated and the intergalactic medium became fully ionized. The bubble growth rate dR/dt is set by the balance of ionizing photons and recombinations: Ṅ_ion = 4π R² n_H Ṙ + (4/3)π R³ α_B n_H² C, where C is the clumping factor. JWST is now directly imaging reionization-era galaxies.