Ronald Reagan Washington National Airport (DCA) sits right in Arlington, accessible via the Blue and Yellow Metro lines. It serves domestic routes from most major US cities.
For international flights, Dulles International Airport (IAD) is about 45 minutes west by car or Silver Line Express bus. Baltimore/Washington International (BWI) offers a third option, reachable by MARC train or shuttle.
If arriving by rail, Amtrak serves Washington Union Station across the river, with Metro connections into Arlington. From the north or south, I-395 and I-66 feed directly into the area, though expect heavy traffic during rush hour.