The concentration of humans – in some cases judged as too high, in others not high enough – and the problems connected to this, have resulted in discussions on density. Prior to the 20th century, density in European cities was merely an outcome of complex circumstances. During the second half of the 19th century, high densities in industrializing cities were argued to be one of the major causes of fires, diseases and social turmoil. In this period, density was introduced as a tool to analyse and diagnose the quickly growing and often overcrowded cities. In a following period of increased state intervention, the concept expanded into an instrument used to propagate alternatives and prescribe maximum densities in order to guarantee certain physiological and social qualities of urban environments (such as air, light and privacy). We can observe a shift from urban density as a mere result of city development and migration to a tool used to analyse problems; and, later on, to an instrument applied to offer improved solutions. More recently, minimum densities are argued for to support amenities and public transport, and as part of the solution to produce more sustainable urban environments with potential for vital human interaction (‘urbanity’). In spite of the practical advantages of the concept of urban density in urban planning, critics have argued that the use of density for anything but statistical purposes is questionable, as it is perceived as a too elastic concept. Many professionals, as well as researchers, hold the opinion that measured density and other physical properties are independent of each other. Besides the argued lack of relationship between density and form, density is also considered with suspicion because of the confusion regarding the definition of plan boundaries and the scale at which these are measured. There is no one accepted measure of density in or shared by different countries. This research takes off with a critical review of the origin, content and practical usefulness of the concept of urban density, and aims at revising and reviving the concept to the benefit of both the planning and design process, and scientific research. This doesn’t mean that an old instrument is just taken out of the basement, dusted off and reignited. No, the shortcomings of the existing density measurements methods in conveying information about urban form and performance are certainly very real, as others have pointed out. Those shortcomings, however, have led many to the conclusion that the concept as such is flawed and even dangerous. We insist, though, that the problem with the most commonly used density measurements methods is one of representation and resolution, and not of the concept itself. The development of the Spacematrix method to measure density and identify a series of associated properties is the main result of this research. We have redefined density as a multivariable and multi-scalar phenomenon to counter the existing Babel-like confusion in the terminology currently being used by those working in the urban field. Further, through the use of this multivariable and multi-scalar approach, density can be related to potential urban form and other performances. This makes it possible to reposition the concept of density in the field of urban planning and design, and research. From an instrument to prescribe the programme of a given area, density can become a tool to guide both quantitative and qualitative ambitions, and thus fuse urban planning and design into true urbanism.