There are a few ways to calculate the critical path. Most begin with drawing an activity diagram to show the sequence and dependancies between the activities. Once the expected duration of each activity is identified it is possible to calculate the earliest each activity could possibly begin. (more later) The longest path is the critical path. If any activity on that path takes longer, the overall timeline will be longer. The critical path can change if the dependancies or activity durations change. The schedule and critical path may also change as the project progresses when actual durations vary from estimates.