How does requirements engineering differ between Waterfall and Agile methodologies?

Waterfall front-loads requirements (gather everything before building); Agile gathers them continuously, just-in-time before each piece is built.

The split reflects a disagreement about whether future needs can be predicted up front.

Waterfall (static):

• All requirements gathered upfront
• Aim to completely elicit and document ALL requirements early
• Done before any design or implementation decisions
• Works when the domain is stable and well understood

Agile (continuous):

• Requirements elicited "on the move"
• Gathered just before they're needed for implementation
• Accepts that predicting future needs is hard, so it defers the decision
• Requirements are expected to change over the project's life

Tip: Waterfall bets that you can know everything early; Agile bets that you can't, so it keeps the cost of changing its mind low.

Go deeper:

• Waterfall model (Wikipedia) — the sequential, requirements-up-front model contrasted against Agile.