Ever since, each hosting nation has picked a clear theme. These have included such broad topics as energy, transportation, art, space, freshwater, the oceans, and the residential environment. World Expos thus evolved from "non-sales product fairs" or "achievement exhibitions" to "thematic expos" with exhibits revolving around one main theme.
從此以後，各國主辦的世博會均有明確主題，範圍包括能源、交通、藝術、太空、淡水、海洋、居住環境等廣泛議題；世博會也從「非交易」的「產品展」或「成就展」，發展成為圍繞單一議題的「主題展」。