ASHOE/MAESA used instruments aboard the ER-2 aircraft to study transport, photochemistry, radiation, and microphysics in the lower stratosphere over a range of seasons and latitudes. During the period from February to November 1994 the ER-2 aircraft flew the suite of instruments between 60 ¿N and 70 ¿S in the longitude sector 115 ¿W to 160 ¿E. Of the 45 flights, 28 were entirely in the southern hemisphere between March and October, 6 were from Hawaii, 2 crossed the tropics between Hawaii and Fiji, and the remainder were from northern California. Data from these and associated ground-based, balloon-borne, and satellite instruments were combined with a variety of operational and research meteorological models to guide the flight planning and to interpret the results. The scientific rationale for the mission is given and the aircraft payload listed. A synopsis of the flights is supplied.¿ 1997 American Geophysical Union