In the case of Microsoft Project 2007, Task Drivers are defined as the factors that affect the Start Date of a particular task.  In Project Professional, on the Menu Bar, under Project, is the option to display Task Drivers.  When a task is selected and Task Drivers are displayed, information about what is affecting that task’s start date is shown.  For clarification purposes, the name of the task and the start date are displayed in the top section of the Task Driver’s pane.

In the lower section of the pane the information is subdivided by different headings with the pertinent information.  If a certain driver doesn’t exist for a task, it isn’t displayed.  For example, if you didn’t apply leveling to your task, then the Leveling Delay driver will not be displayed.

The potential drivers and causes for why they might appear are:

Driver                                                                   Cause                                                   

Actual Start and Assignments                     Actual data was entered

Leveling Delay                                             Resource leveling was done

Constraint Type/Date                                  A constraint is set

Summary Task                                              A constraint is set on a task’s summary task

Predecessor Tasks                                       Linked tasks

Subtasks                                                       A summary task will list the subtasks

Calendar                                                       If a calendar is affecting the task (may display                                                                      the Resource name because of their calendar

Examples:

A simple example of this is explained here.  For the task selected, task id 6 “Secure Core Resources” we see the task information in the top part of the pane.  In the bottom part of the pane, we see the predecessor task and that the calendar for the resource “Ann” is affecting the start date.

In this example, task id 9 has a predecessor but the real driver on the start date is that a “Start no earlier Constraint” has been set and the calendar being used for the resource is affecting it.