What distinguishes Waterfall from Agile approaches in handling changing requirements?

Waterfall locks requirements in up front and resists change; Agile gathers them iteratively and welcomes change every sprint.

* Waterfall locks requirements up front; Agile loops each iteration and welcomes change. *

Waterfall:

• All requirements gathered upfront
• Changes are expensive and difficult
• Works when requirements are stable and well-understood
• Sequential phases with clear milestones

Agile:

• Requirements gathered iteratively
• Changes are welcomed and expected
• Works when requirements are evolving or uncertain
• Continuous delivery of working software

Security implication:

• Waterfall: Security requirements defined early, less flexibility
• Agile: Security must be integrated into each sprint, continuous security testing