A baseline schedule is the contractor's initial CPM schedule, formally accepted by the owner, used as the reference point for all subsequent progress and delay analysis. It locks in the planned duration and sequence of every activity at the start of the project. Once accepted, the baseline is preserved unchanged. Updates happen on a current schedule (sometimes called the working schedule) that is compared back to the baseline.
Baseline schedules matter most when delay claims happen. Without a baseline, there is no objective benchmark for what was supposed to happen. Most modern commercial contracts require baseline submission within 30 to 60 days of notice to proceed and require monthly updates throughout the project. Skipping the baseline submission or accepting a baseline without owner review is a common source of delay-claim weakness later.