  NASA's Chandra X-ray Observatory and ESA's XMM-Newton made their first
observations one decade ago. The unprecedented and complementary capabilities
of these observatories to detect, image, and measure the energy of cosmic
X-rays, achieved less than 50 years after the first detection of an extra-solar
X-ray source, represent an increase in sensitivity comparable in going from
naked-eye observations to the most powerful optical telescopes over the past
400 years! In this review, we highlight some of the many discoveries made by
Chandra and XMM-Newton that have transformed 21st century astronomy and briefly
discuss prospects for future research.
