Step 1: Activity Definition involves the identification and definition of those activities that must be accomplished to achieve the objectives of a project or program. The Work Breakdown Structure (WBS) is a logical source for such descriptions. If a WBS is not available, more planning must be done in order to identify project activities clearly. Other inputs to the definition step are the program charter, scope, historical information, program constraints and assumptions, and events required by the DoD Planning, Programming, Budgeting and Execution (PPBE) Process, the JCIDS ”Requirements Generation” Process, and Acquisition Process.
Techniques commonly used in activity definition are: 
- Decomposition involves the successive breakdown of program elements into smaller, more manageable components, which eventually describe the activities to be scheduled. This technique is essentially the same used in WBS development.
- A template is an activity list or WBS element from another similar program that can serve as a model for the current program and provide a starting point for defining specific activities.
The primary output of this step is the activity list, which should contain a complete description of each of the activities necessary to complete the program. This list should be linked to the WBS, which should be reviewed and revised/clarified as necessary to incorporate changes resulting from the activity definition process. Supporting details for each activity, such as constraints and assumptions, should also be developed and documented. 
Schedule Development Steps
- Step 1: Activity Definition
- Step 2: Activity Sequencing
- Step 3: Activity Duration Estimation
- Step 4: Schedule Development
- Step 5: Schedule Control
AcqLinks and References:
- Defense System Management College “Scheduling Guide for Program Managers” – Oct 2001
- DoD “Integrated Master Plan and Integrated Master Schedule Preparation and User Guide” – 21 Oct 2005
- CDC “Project Scheduling Best Practices/Guide” – 30 Jun 2007
- GAO 12-120G “Schedule Assessment Guide: Best Practices for Project Schedule” – May 2012