[1] He then worked as an associate professor at Sofia Technical University and entered politics after the collapse of the Berlin Wall and the fall of the Bulgarian communist leader, Todor Zhivkov.

The UDF won the May 1997 elections, and Kostov became Bulgaria's Prime Minister, his cabinet eventually became the country's first post-communist government to serve its full 4-year term.

Kostov is credited with turning around his country's fortunes, implementing the currency board in Bulgaria, removing price controls and creating a modern market economy, which put it on the path of sustainable economic growth.

Against the insistence of the United States, Kostov didn't allow refugees from Kosovo to enter Bulgaria, but arranged for them to be temporarily settled in the Republic of Macedonia instead.

[3] In 2010, the GERB government of Prime Minister Boyko Borisov proposed instituting a tight fiscal rule in the Bulgarian Constitution, namely that the budget deficit could not exceed 2% of GDP in any one year.

Kostov's DSB supported several other initiatives by GERB's government, including the ban on smoking in public places, and the pension reform proposed by Finance Minister Simeon Dyankov.