How easy it is to enter and stay in Bulgaria depends on your country of origin. If you are from a country member of the EU, the European Economic Community (EEC) or from Switzerland, Canada, the US and Israel, you do not need a visa to enter the country. You may stay in the country for up to 90 days without any additional documents required, counting from the first date of entry. After the end of the three-month period you will need to apply for long-term or permanent stay permits.
Travellers who are not citizens of the countries listed above should obtain an entry visa in advance from the Bulgarian embassy or consulate in their home country. A letter of invitation from a Bulgarian host, in a standard format or a letter from company or institution proving business travel is needed for the issuing of Bulgarian visa. 
Visa applicants should note that incomplete forms, absence of photographs or inadequate payment usually results in the return of the application.