For visits of up to 90 days visas are not required for most Western nationals, including Americans, citizens of EU countries, Australians, Canadians, New Zealanders, Malaysians, Singaporeans and most South Americans.
Residents of other countries should check requirements with a Swedish or Danish embassy. When a visa is required, only one visa (either Swedish or Danish) is needed to visit both countries. For instance, the holder of a Swedish visa has the right to arrive via Copenhagen Airport without breaking any visa regulations, and vice versa.
The invitation letter will be issued from the host country of the congress. Please note an invitation letter will only be issued to those participants who have paid registration fee in full.