A critical path is the longest path in a project plan. There is no slack in any task on the critical path, so if there is a delay in any of these tasks, the completion of an entire project will be delayed. Consequently, a project manager’s attention is heavily focused on ensuring that each task along the critical path begins and ends on time.
It is possible that there are several near-critical paths associated with a project. If the tasks within these paths become prolonged, it is entirely possible that one of the paths will become the new critical path, supplanting the original one. If this happens, the attention of the project manager is then focused on the new critical path. When there is a near-critical path, it behooves the project manager to watch it closely over time and evaluate the risk that it may become the new critical path.
The critical path concept is a key element of the program evaluation and review technique.