Many consider Chicago the birthplace of the skyscraper, and Chicago architects continue to lead the field in tall building design both at home and abroad. Tall buildings offer spatial efficiencies in a world that grows more urban by the day, and today's skyscrapers are breaking new ground in height, innovative design, and optimal densities. AN peels back the skin andddesign processsof four projects currently underway by prominent Chicago offices: Studio Gang's stunning, rippled Aqua tower, north of Millennium Park; DeStefano + Partner's naturally ventilated supertall Zenith Towers in South Korea; the muscular twist of SOM's Infinity Tower in Dubai; and Adrian Smith + Gordon Gill's retrofit of Chicago's iconic Willis Tower.