(1) We suggest a theoretical framework for considering limits to relatively stable distributions and illustrate some of the points raised with information on the distribution of two species of kangaroos. (2) If an attribute such as density or condition is low at the periphery but rises progressively towards the core of the distribution, its trend is described as a `ramp'. If the level of the attribute differs little between the periphery and the core of the distribution its trend forms a `step' at the range boundary. (3) Should density form a ramp inwards from the boundary whereas the mean well-being of the animals (e.g. body condition, growth, weight, recruitment) forms a step, the factor limiting distribution is likely to be a resource that is utilized consumptively or pre-emptively. (4) Should both density and well-being form a ramp, the implicated factor is a component of climate, an unmodifiable resource, or a facultative predator, parasite or pathogen. (5) Should both density and well-being step at the range boundary, the factor controlling the position of the boundary is likely to be the substrate (e.g. a rock type). (6) Two kangaroo populations were sampled at the core and periphery of their respective ranges. The southern (=`western') grey kangaroo Macropus fuliginosus (Desmarest) exhibited a ramp of both density and well-being which, in combination with ecological information on this species, suggested that the edge of the range was positioned by a component of climate perhaps interacting with an unmodifiable resource. (7) The eastern grey kangaroo Macropus giganteus Shaw exhibited a ramp of density but a step of well-being, implicating a renewable resource as the factor determining the inland boundary of distribution. (8) Density and distribution are likely to be different aspects of the same thing except where the limiting factor or combination of factors is or includes a renewable resource consumed by the animals.