ABSTRACT. Local reasoning about programs exploits the natural local behaviour common in programs by focussing on the footprint -that part of the resource accessed by the program. We address the problem of formally characterising and analysing the notion of footprint for abstract local functions introduced by Calcagno, O'Hearn and Yang. With our definition, we prove that the footprints are the only essential elements required for a complete specification of a local function. We formalise the notion of small specifications in local reasoning and show that, for well-founded resource models, a smallest specification always exists that only includes the footprints. We also present results for the non-well-founded case. Finally, we use this theory of footprints to investigate the conditions under which the footprints correspond to the smallest safe states. We present a new model of RAM in which, unlike the standard model, the footprints of every program correspond to the smallest safe states. We also identify a general condition on the primitive commands of a programming language which guarantees this property for arbitrary models.