Create a milestone trigger

Create a milestone trigger using an escalation map or an alert task.

Use an escalation map

Use an escalation map to create a specific job to deal with the milestone trigger.

In the New milestone dialog box, click the Triggers tab.
Click 
On the Process list, select a process to use as an escalation process that will take some corrective action.
To set the trigger to execute later than or before the milestone due date, set the Target duration in days, hours, minutes, and/or seconds and then select After or Before. For example, to execute the trigger three days before the milestone is due, set 3 days for the target duration and select Before.

To execute the trigger when the milestone due date is met, keep the target duration value 0.
Click Add.

Create an alert task using milestones to appear on the work queue of the selected resource to notify escalations.