Brownfield is a polysemic notion that encompasses a whole range of diverse spaces. Although there is no official definition at the European level, an emerging consensus has arisen around the urban character of brownfields and the need for intervention. Indeed, their location within metropolitan areas represents a strategic opportunity to densify and rejuvenate the urban fabric at the neighbourhood scale. Hence, we propose a definition of urban brownfields that is flexible enough to optimize the potential development of abandoned sites and precise enough to enable framing the discussion. Then, we take a look at the diversity of urban brownfields in European metropolitan areas by briefly explaining the factors that cause a site to become a brownfield and then attempting to classify different types of urban brownfields. The proposed classification aims to create a non-exhaustive reference framework by offering in-depth knowledge of the urban brownfield and regeneration projects phenomenon across Europe, without setting its meaning in stone.