Waterfall

Winston Royce described what became known as waterfall in a 1970 paper on managing large software projects. Ironically, Royce's paper actually argued that the sequential model was flawed and recommended iterative development — but the sequential model stuck and became the dominant approach in defense and enterprise software through the 1980s and 1990s.

The five classic phases:

1. Requirements: Gather and document all system requirements before design begins
2. Design: Create the system architecture and technical design from the requirements
3. Implementation: Write code according to the design
4. Testing: Verify the system meets requirements
5. Deployment: Release to production and maintain

The fundamental problem with waterfall is the assumption that requirements can be fully known and frozen at the start of a project. In practice, requirements change, business conditions shift, and what made sense in Phase 1 doesn't make sense by Phase 4. Because testing comes so late, defects discovered there are expensive to fix — the code may need to be redesigned, not just patched.

Waterfall remains appropriate in some contexts: regulated industries where requirements are fixed by law or contract, hardware development where physical constraints limit iteration, and projects where the domain is extremely well understood. In software product development — where learning and adaptation are competitive advantages — agile methods have largely replaced it.

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