Waterfall is a traditional project management methodology where projects proceed through sequential phases—requirements, design, implementation, testing, deployment—with each phase completed before the next begins. Waterfall assumes requirements can be fully defined upfront and changes are costly once development begins. This approach suits projects with stable, well-understood requirements and regulated environments requiring documentation at each stage. Government IT projects have traditionally used waterfall approaches, though limitations in handling changing requirements have driven adoption of agile alternatives. Waterfall provides predictability and clear milestones but struggles with projects where requirements evolve or aren't fully understood initially. Many organizations now use hybrid approaches combining waterfall structure for overall project governance with agile methods for development work.