The concepts of ecological planning and environmental planning continue to be surrounded with confusion. The present paper aims at defining the first, ecological planning, and at showing its specificity in relation to the second, environmental planning. It demonstrates how the idea of externality links fundamentally the two concepts but suggests that the preponderance given to harmonizing development with the natural environment conveys its specificity to ecological planning within environmental planning. The full meaning of this harmonizing and its relevance in actual land use and development planning are then explored ; this harmonizing is rooted both in a vision of interdependence between the economy and the natural environment but more so in a desire to locate our development strategies within the framework of our responsibility towards future generations. Far from being the results of transient preoccupations, ecological planning would seem to bring in the foreground and to restructure some concerns deeply imbedded in the conservation movement.