Entry and Visa
Bulgaria has been a member of the EU since 2007, however it is not a signatory to the Schengen Treaty. They are the only EU countries that are not yet members, along with Romania. This means that you'll need your ID to enter Bulgaria, even if you're from the EU. In any case, Bulgaria's visa requirements are identical to those of other EU countries; click here for more information!
Although Bulgaria is a member of the European Union, it is not yet a member of the Schengen Area, which provides for free movement between countries and which many EU members now enjoy. This implies that you will have to go through passport control while entering Bulgaria, and visa requirements differ. For visa information, see here, but at the time of writing, visitors from Australia, New Zealand, the United States, Canada, and most of Europe did not need visas for stays of less than 90 days.
Holders with valid multiple Schengen visas are exempt from visa requirements in Bulgaria. They will be able to enter and stay in the Republic of Bulgaria for a maximum of three months in any six-month period following their initial arrival, without the need for a Bulgarian short-stay visa. This is definitely one of the top Things to Know Before Traveling to Bulgaria.