The Precedence Diagramming Method (PDM) is a classical technique for project scheduling where activities have four types of precedence relations (start±start (SS), start±fnish (SF), fnish±start (FS) and fnish±fnish (FF)) and assumes that an unlimited quantity of all necessary resources are available for assignment to individual activities whenever they are requiredProject managers generally consider slack as a measure of the scheduling flexibility associated with the activities in the project network. Nevertheless, when resource constraints appear, this information must be calculated and analysed carefully. In this context, to handle project feasible schedules is very hard work for project managers. In order to develop useful tools for decision making, the authors extend the concepts of activity slack and define a new activity criticality index based on them that permits us to classify the activities in the resource-constrained project scheduling and control context. Additionally, these new concepts have been integrated into standard project management software as new commands. Hence the capabilities of project management software are improved.