This chapter examines multidisciplinary views on the effects of density on health between 1920 and 1980. This was an era in which the originally close connection between planning and public health became broken, but presumptions regarding the effects of density on health still influenced urban planning standards and ideals, contributing to a decline in urban densities. While progress in medicine and urban sanitation reduced density’s significance with respect to infectious diseases, the density problem also acquired new facets, for instance, in relation to stress-related diseases, ranging from blood pressure to mental ailments. The chapter argues that in the late 1960s and in the 1970s, the density issue was re-evaluated in many scientific disciplines, including epidemiology, and was particularly well scrutinized in environmental psychology and urban sociology. This re-evaluation strengthened pro-density trends in urban planning and allowed new kinds of thinking about density and urbanization in the coming decades.