Montgomery County is the most populous county in the state of Maryland. Montgomery County, which adjoins Washington, D.C., is part of the Washington–Arlington–Alexandria, DC–VA–MD–WV metropolitan statistical area, which in turn forms part of the Baltimore–Washington combined statistical area. The average household income in Montgomery County is among the highest in the United States. It has the highest percentage (29.2%) of residents over 25 years of age who hold post-graduate degrees. Like other inner-suburban Washington, D.C. counties, Montgomery County contains many major U.S. government offices, scientific research and learning centers, and business campuses.