How Do Resource Overallocations Happen?

When you schedule resources to work on tasks in your project, you almost inevitably create some "impossible" resource workloads. Microsoft Project calls it an overallocation when a resource is assigned to more work than can be realistically completed.

Overallocation is a term that's used to refer to the situation where a resource's assignments for a given time period call for more hours of work than the resource has available to deliver. The resource is said to be overallocated during that time period. The situation is created when the sum of the resource units assigned to all tasks during that time period is more than the maximum number of units available for the resource for that time period. The maximum units available for a resource during any time period is defined in the Resource Information dialog box.