The Critical Path is the longest path of scheduled activities that must be met to execute a project. This is important for Program Managers (PM) to know since any problems on the critical path can prevent a project from moving forward and be delayed. Earned Value Management (EVM) analysis focuses on the critical path and near-critical paths to identify cost and schedule risks. Other schedule paths might have slack time to avoid delaying the entire project, unlike the critical path. There might be multiple critical paths on a project.
The Critical Path is determined when analyzing a project’s schedule or network logic diagram and uses the Critical Path Method (CPM). The CPM provides a graphical view of the project, predicts the time required for the project, and shows which activities are critical to maintaining the schedule.
The seven (7) steps in the CPM are: 
- List of all activities required to complete the project (see Work Breakdown Structure (WBS)),
- Determine the sequence of activities
- Draw a network diagram
- Determine the time that each activity will take to completion
- Determine the dependencies between the activities
- Determine the critical path
- Update the network diagram as the project progresses
The CPM calculates the longest path of planned activities to the end of the project and the earliest and latest that each activity can start and finish without making the project longer. This process determines which activities are “critical” (i.e., on the longest path) and which have “total float” (i.e., can be delayed without making the project longer). 
The CPM scheduling technique was introduced at approximately the same time as PERT Analysis. It was developed by J. E. Kelly of Remington-Rand and M. R. Walker of DuPont to aid in scheduling maintenance shutdowns in chemical processing plants. Over the years, CPM has enjoyed more use than any other network scheduling technique. It is based on the concept of critical path and was designed to focus on the time and resources, particularly cost, necessary to complete a project’s activities.
Although CPM and PERT are conceptually similar, some significant differences exist mostly due to the type of projects best suited for each technique. As discussed earlier, PERT is better to use when there is much uncertainty and when control over time outweighs control over costs. PERT handles uncertainty of the time required to complete an activity by developing three estimates and then computing an expected time using the beta distribution. CPM is better suited for well-defined projects and activities with little uncertainty, where accurate time and resource estimates can be made. The percentage of completion of an activity then can be determined. 
AcqLinks and References:
-  Defense Systems Management College “Scheduling Guide for Program Managers” – Oct 2001
- Website: Project Manager.com – Ultimate Guide to the Critical Path Method