Seizing the title of "world's tallest building"-be it Joseph Pulitzer's 20-story New York World newspaper building in lower Manhattan in 1890, the Empire State Building in 1931, or Kuala Lumpur, Malaysia's gargantuan Petronas Towers in 1998-has always been about pushing the limits of architecture and engineering. But three years after the attacks of September 11 demonstrated how vulnerable such buildings are to terrorists, a surprising new competition is under way. The latest skyline king is a vaguely pagoda-like tower in Taipei, Taiwan, called Taipei 101 (see "Inside Taipei 101," below). Slated for occupancy this fall, the 101-story structure stands 508 meters tall, more than half a football field higher than Petronas. On the horizon are even taller skyscrapers, including the Freedom Tower proposed for New York City's ground zero, and a business and residential colossus in Dubai, United Arab Emirates.

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