10 Metaverses 10.1 Introduction Despite what many readers may think, the metaverse is not a product, not even a brand of some social network company, but a name given to a set of technologies applied in platforms for the Web on the Internet. In fact, the concept of virtual worlds dates back at least to the 19 th Century [21]. Still, the term metaverse was used to name a futuristic concept, described in a science fiction book in 1992, which popularised it [198], and was shown in a visual format in a movie 20 years later (i.e. 10 years ago) [140]. Metaverse in practice today, refers to a new type of Web platform, which is supported through a comprehensive set of technologies, some of which already consolidated and others in evolution, which will allow users greater interactivity and socialisation in immersive 3D digital environments, represented by a universe of new digital virtual worlds, preferably mirrored in the physical world. Metaverse research witnessed a first wave of "hype" between the years 2000 and 2006, with many results and visibility. Currently, in 2022, it is going through a second wave of interest, now brought about by commercial players that started to market their metaverses and events held inside them, but also by a widely publicised metaverse-related public announcement by one of the Western Big-Techs in late 2021. Today, it is possible to understand what metaverses are, or could be, by browsing through Web platforms such as Second Life, Decentraland, Somnium Space, The Sandbox, Roblox, Horizon Worlds, Avakin Life, Mesh, among others. The concepts of digital virtual worlds of today's metaverses are typically based on Web 2.0 technologies that include 2D and 3D Virtual Reality spaces, with computer graphics images ranging from low to high resolution, and some platforms using Augmented Reality technologies in various activities. The representation of users is always through avatars, and, as access to platforms depends on an exclusive login, there is a lack of interoperability, as avatars are confined to a single metaverse and its worlds, not being allowed