Terraria's progression is gated by boss kills. Each tier unlocks new gear, biomes and mob types — so always fight bosses in order. Use the per-boss "How to beat" guides for spawn conditions, drops and the recommended progression slot.
Fight them in progression order — each boss unlocks new gear, ores and biomes. The path is roughly: pre-hardmode bosses (King Slime, Eye of Cthulhu, Eater of Worlds or Brain of Cthulhu, Queen Bee, Skeletron) → Wall of Flesh to enter Hardmode → the three Mechanical Bosses → Plantera → Golem → Lunatic Cultist and the Celestial Pillars → Moon Lord.
This checklist tracks 31 bosses across every progression tier, from King Slime to the Moon Lord.