Lead Time vs Lag Time

Sometimes a task with a dependency needs to start sooner than one (or more) of its predecessor tasks can be finished. This is represented using lead time and lag time.

Lag	The amount of time in the schedule that a task can be delayed before it will affect the scheduling of other tasks. Lag occurs when there is a delay between two tasks that share a dependency.
Lead	Lead time occurs when there is overlap between two tasks that share a dependency.

Lead time and lag time are entered as a duration or as a percentage of the task's duration, where a negative entry is used for lead time and a positive entry is used for lag time. For example, -1d is a lead time of one day and 3d is a lag time of three days. Use a percentage to indicate at what point the lead time or lag time should begin. For example, a lead time of -50% indicates that a task may begin when its predecessor is 50% complete.

It is important to note that lead time and lag time is not the same as slack, though (at times) they can appear in a similar manner in the schedule. Lead time and lag time are deliberately scheduled events in the task plan, whereas slack indicates room (or lack of room) in the task plan's schedule.