Usually when you hear the words "succession planning" it is in the context of a privately held or family-owned company.  However, this principle should also apply to every level of management within your organization.

What person or persons have the potential to move into each manager's position should the need arise?

Of course not ever internal position can be or should be filled completely from within.  But before that choice can be made, senior leadership should have a "scorecard" that outlines the talent available.  This scorecard should also be used to foster the development of key individuals by challenging them with varied tasks and responsibilities to increase their management experiences.

If you haven't assessed your internal management talent lately, it should be a top priority.  You never know when that key stategic void will occur.