Fifteen years ago, on 25 August 1997, NASA launched the Advanced Composition Explorer (ACE) spacecraft. In operation for more than a solar cycle, ACE has provided numerous important scientific results while at the same time becoming a key component of the space weather–monitoring system and a cornerstone of the fleet of distributed spacecraft that make up the Heliophysics Great Observatory. ACE is located on the sunward side of Earth about 1.5 million kilometers away, or 4 times as far from Earth as is the Moon. This puts ACE well outside the Earth's magnetosphere and in an ideal position to monitor the solar wind environment “upwind” of Earth. ACE has six high-resolution spectrometers that measure the elemental, isotopic, and ionic charge-state composition of ions from hydrogen to iron; it also includes three particle and field monitors. One of the nine instruments has now failed (Solar Energetic Particle Ionic Charge Analyzer (SEPICA)), and two others are partially degraded (Solar Wind Ion Composition Spectrometer (SWICS) and Solar Wind Electron, Proton, and Alpha Monitor (SWEPAM)); however, the other six are working well. Instrument descriptions are available on the ACE Web site at http://www.srl.caltech.edu/ACE/.