describes the history, development, and role of virtual worlds, also known as virtual environments and immersive virtual environments. It provides detailed background about virtual worlds and their societal impact, from early precursors and inspirations to the latest trends and developments. Specifics on user demographics are included, as are descriptions of virtual worlds' functions, discussion of societal concerns and opportunities, and information about relevant research data and key persons and organizations. Although virtual worlds in their current form are a relatively new phenomenon, other online social environments have served as precursors for decades and literary inspirations go back even further. This handbook therefore covers some early developments dating back to the mid-20th century. Its primary focus, however, is on developments since the mid-1990s and especially on the current state and social impact of virtual worlds, including their impact both in the United States and around the world.