The global transition from conventional battery cages to cage-free (barn, free-range, aviary) systems represents the largest structural change in layer hen production in decades. Welfare outcomes in cage-free systems are mixed: hens can express more natural behaviours (dustbathing, perching, wing flapping) but face elevated risks of feather pecking, injurious pecking, keel bone fractures, and increased disease transmission in poorly managed flocks. Research shows keel bone fractures affect 50-80% of hens in aviaries - a significant welfare finding. System design (perch height, nest provision, litter quality, lighting) substantially affects outcomes. Aviary management requires higher stockmanship skill than cages. Countries and retailers specifying cage-free have driven 40-60% market transitions in the US and EU. Welfare science emphasises that system type matters less than management quality in determining welfare outcomes.