Abstract
This paper is concerned with the electron density and other ‘property densities’ and, through the use of concrete examples, touches upon some of the fundamental aspects of density functional theory (DFT) that are seldom considered. In particular, this paper considers what density functions are needed in discussing observable electronic properties and what is their role in DFT. The implications of the Pauli Principle and the constraints imposed by the requirement of N-representability are considered in some detail.