ACHIEVEMENTS of the last ten years in increasing the power-weight ratio of aircraft engines are stated and contributing factors are analyzed. Aluminum alloys have replaced cast iron and steel for certain parts, not entirely because of their lower weight but because of a combination of properties which better fit them for the task. Similar considerations must govern the replacement of aluminum-base alloys by those of magnesium. The most promising immediate field for the magnesium alloys is said to lie in applications wherein strength and lightness are the main considerations and high-temperature properties are of secondary importance.Properties of magnesium castings and forgings are compared with those of castings and forgings of the aluminum alloys. Features of design are discussed which should receive special attention when changing a part from aluminum to magnesium.Machining practices for magnesium are covered in some detail. The author states that the present casting-alloys have proved commercially satisfactory as to corrosion for most types of land service. In saltwater service, especially in tropical countries, a certain amount of trouble has been encountered and some protective measures seem to be necessary. A recommended treatment for protection against corrosion is offered.Discussion of the paper touched on corrosion, including the effects of contact with wood and with gaskets; on machining and heat-treatment; and on the rapid improvement that is being made in magnesium.