The International Year of Astronomy 2009 (IYA2009) was conceived to honor the 400th anniversary of the first use of an astronomical telescope by Galileo Galilei in 1609, and has evolved into an engaging series of worldwide programs. IYA2009 is sponsored by the International Astronomical Union (IAU) and endorsed by the U.S. House of Representatives, UNESCO, and the United Nations. During this year, we are making great efforts to stimulate worldwide interest in astronomy and science, especially among young people and underserved populations. More than 145 countries and three dozen agencies are participating in this global effort, with the United States taking command of three large cornerstone projects described in this article, assisted by funding from the National Science Foundation and leadership from the American Astronomical Society and NASA.