The late 19th and early to mid 20th Century marked the zenith of the popularity of large expositions that brought together artists, industry, and consumers. There were regional expositions, national expositions, and of course the highly anticipated World's Fairs (see links below).
Each exposition allowed participating artists and artisans to compete for prizes and to demonstrate their talents in front of well-disposed crowds. Manufacturers and inventors showcased their new products, hoping they would resonate with the visitors. For the large World's Fairs millions of visitors toured the fair grounds and buildings that were often custom-built just for the event.
After World War II the popularity of the World's Fairs started to decline while specialized industry fairs became more widespread and advertisement dollars started shifting from events to TV. While there are still World's Fairs being held, the advent of the internet and the ability to view things remotely has further eroded their popularity and relevance.
The big expositions usually had prize medals for exhibitors, medals that were designed by some of the greatest medallic artists of their time. In addition to these "official" medals there were usually many exhibitor-issued tokens, coins, medals, and medallions. The best-known expositions produced so many collectable exonumia that they attract their own specialty collectors.
The list below provides links to pages about particular expositions. I am not an expo specialist. If you are a specialist you can greatly help by providing me by submitting material or by correcting mistakes I might have made as an expo novice.
Contact me if you have links that might merit inclusion on this page.