The responsibility for drafting and maintaining the Project Management Plan (PMP®) usually lies with the Project Manager.  However, he/she does not do so in isolation.  It is important that all members of the project team collaborate with the PM regarding the creation of the Project Management Plan.  The Project Manager should also seek input from key stakeholders.  This input is critical since a successful Project Management Plan requires proper identification of the effort, definition of staffing needs and other resource requirements as well as estimation of the time required of staff.