You should choose the Waterfall methodology over Scrum when a project has fixed, well-defined requirements that are unlikely to change during the development lifecycle. Waterfall is a linear, sequential approach where each phase must be completed before the next begins, making it ideal for industries where the cost of "rework" is extremely high, such as construction, hardware manufacturing, or aerospace. In 2026, it is also heavily used for highly regulated compliance projects (like government or finance) that require extensive upfront documentation and rigid audit trails. Conversely, Scrum is an "Agile" framework best suited for complex, evolving projects like software development where user feedback can change the product's direction every few weeks. A grounded rule of thumb: if you can clearly see the "finish line" from the start and the path to get there is a straight line, use Waterfall. If you are "innovating" or building something where the requirements will emerge over time, Scrum is the more supportive and flexible choice for your team.