Entry and Residence
If your nationality is from a state of the European Union, the European Economic Area, or Switzerland, you are allowed to enter and study in Austria with a valid travel document without any visa or residence permit. You have to register with the municipal authorities within three working days after entering Austria. If you stay in Austria for longer than 90 days, you also have to apply for a confirmation of registration.
If your nationality is from a different state, you must apply for a residence permit for study purposes ("Aufenthaltsbewilligung Studierender") at the Austrian representative authority before entry to Austria and after receipt of the notification of admission from the university. The application must be sent to Austria from your home country and the decision has to be awaited (therefore the application should be submitted at least five months before the intended arrival in Austria).
For detailed and up-to-date information on entry to and residence in Austria, including the list of required documents and procedures, read the Study Guide (Section "Entry and Residence Conditions") of the portal Study in Austria maintained by the OeAD.