Thispaper describesAPE(the Atlas PlanningEngine),the reactive planner at the center of the Atlas dialogue managementsystem. The goal of Atlas is to build conversation-basedsystems, whereturns in the "conversation" mayinclude graphical actions and/ortext. Since APEcan be used to generate a dialogue involving arbitrarily nesteddiscourseconstructs,it is morepowerful than dialogue planners basedon finite-state machines. Although it is intended largely to model dialogue containinghierarchical, multi-turnplans, APEcanalso be used as a general-purpose programmingtool for implementinga dialoguesystem.