“Expo Plovdiv 1991” was held in Plovdiv, Bulgaria, from June 7 to July 7, 1991. A specialized exhibition under the Second Categorization System, it was the second edition of the international exhibition of young inventors that Plovdiv had first hosted in 1985. It was organized by Bulgaria's Ministry of Economy and Planning, the Chamber of Commerce and Industry of Bulgaria, and several national bodies for youth creativity and invention, in cooperation with the World Intellectual Property Organization (WIPO). The BIE gives its theme as “Youth Inventiveness for a Peaceful World.”

At the request of the Bulgarian government, the BIE's 104th General Assembly recognized the Expo on December 14, 1988. It did so despite a moratorium on new Expos, other than horticultural exhibitions, for the years 1989 to 1991, citing the quality of the project. It was Plovdiv's third Expo, after Expo 1981 Plovdiv and Expo 1985 Plovdiv. By the time it opened, Bulgaria was in the midst of the political and economic reforms that followed 1989, and the Expo was intended to support that transition in Bulgaria and its neighbors.

Eight countries took part officially, while 36 countries were represented through individuals, companies, institutes or universities, along with 16 international organizations. More than 3,000 inventions and innovations were presented in 14 thematic groups, from electronics and aerospace to chemistry, medicine, energy and food, with projects on ecology and tourism as well. Exhibitors had to be under 45. About 10% of the projects came from schoolchildren under 18, 22% from people aged 18 to 25, and 59% from those aged 25 to 35. An international jury awarded 187 gold medals and 40 special prizes.

More than 180 scientific and cultural events accompanied the exhibition, including 45 conferences. From July 3 to 5, the Bulgarian government and WIPO held a three-day international symposium on young inventors and innovators, attended by representatives of 25 countries. In all, 140 reports and speeches were presented on the theme of invention.

The Expo's main physical legacy was the expansion and modernization of the grounds of the International Fair Plovdiv, including four new pavilions, which strengthened the city's conference and exhibition industry and its role in international trade. Expo 1991 Plovdiv placed the creativity of young people at the heart of an Expo at a moment of deep change for Bulgaria.