As Bulgaria is a member of the EU, citizens of other EU nations will face minimal border formalities.
Delays are common at border crossings, and customs officials are generally an unfriendly and suspicious lot; expect to be questioned on what business you have in coming to Bulgaria and where you intend on staying.
There are no restrictions on any foreign-passport holders entering Bulgaria, other than the length of time they are allowed to stay. Passports should be valid for at least three months after the date of your intended departure from Bulgaria.
Visas are not required for EU citizens. Citizens of Australia, Canada, New Zealand and the USA can visit visa-free for up to 90 days.
Citizens of other EU member states and Australia, Canada, Israel, Japan, New Zealand, the USA and several other countries can stay in Bulgaria visa-free for up to 90 days. Other nationals should check the current requirements with their nearest Bulgarian embassy or consulate before their departure. Visas cannot be obtained at border crossings. At the time of writing, Bulgaria was not a member of the Schengen zone, and its application to join had long been pending.
Visitors wishing to extend their visit to Bulgaria beyond the 90-day limit have to apply for a residence permit at the National Migration Directorate (http://migration.mvr.bg); application forms can be downloaded from the website. This is likely to be a time-consuming, bureaucratic nightmare, and nobody here will speak anything but Bulgarian. It’s probably far better to contact the Bulgarian Embassy in your own country for advice before you travel if you envisage being in the country for more than three months. The Bulgarian Ministry of Foreign Affairs (www.mfa.bg) has useful information, in English, on visas and other immigration matters.