EEA, Monaco and Switzerland
If you are a national of an European Economic Area country (EU + Norway, Iceland and Liechtenstein) or from Monaco or Switzerland you may travel to Belgium on the strength of a national passport or identity card.
Any other nationality
If you have any other nationality, you have to apply for a visa (type short stay) with the Belgian embassy or consulate in your current country of residence before leaving for Belgium. Upon acceptance to a summer school, you can request a visa invitation letter through Mobility Online.
More info on visa requirements
For some countries, approval by the Belgian authorities can take up to three months. Make sure you start the visa application procedure well in advance!