There are two main airports in Belgium: (1) the National Airport of Brussels Zaventem and (2) Brussels South Charleroi (mainly for budget airlines). We recommend arriving at Brussels Zaventem Airport because it offers easier and faster connections by rail to Leuven. A direct train connects Brussels Zaventem Airport to Leuven in 15 minutes and costs 5.40 EUR. They run twice every hour (at the 17' and 39' minute mark).
Connections from Brussels South Charleroi Airport to Leuven involve a combination of bus and train, entailing a journey of at least 90 minutes, which costs roughly 20 EUR. The other option is a Quick bus which travels 10 times a day from the airport to Leuven. Online ticket sale at least 48 hours inadvance: http://www.brussels-city-shuttle.com/en/#/
Important: Be careful not to confuse Leuven and Louvain-La-Neuve as these are two different towns. If in doubt, please ask at the ticket booth or consult the NMBS website for detailed information.
Leuven can easily be reached by car. The E40 highway connecting Brussels and Liege runs through Leuven. The E314 highway links Leuven with the province of Limburg and from there on to Maastricht and Aachen, all in about one hour. There are different car parks in Leuven to choose from.
Please find in attach a printer-friendly document of the travel information (pdf):