Several mixtures of phospholipids and cholesterol produce coexisting liquid phases when deposited as a monolayer at an air-water interface. For model mixtures near the miscibility critical point, distinctive `stripe' phases are observed. Stripe phases are also observed in monolayers with lipid compositions approximating the inner or outer leaflets of human red blood cells. For monolayers containing cholesterol and lipids with high melting temperature, the phase diagram can exhibit two upper miscibility critical points rather than only one.