What is the critical path method and why does it matter?

The critical path method identifies the longest sequence of dependent activities that determines the minimum project duration. Any delay on a critical path activity directly delays the project completion date because these activities have zero float or slack. Non-critical activities have float, meaning they can be delayed without affecting the project end date. To find the critical path, perform a forward pass calculating early start and early finish dates, then a backward pass calculating late start and late finish dates. The difference between early and late dates is the float.