A design hypothesis is crucial in the UX and UI design process as it guides research, influences design solutions, and maximizes learning through iterative design. A well-defined hypothesis provides context, helps prioritize research by focusing on areas with less confidence, and enables designers to make informed decisions. It also outlines the objective of design changes and maps underlying assumptions, which can be validated to avoid pitfalls and refine solutions post-launch. Examples of design hypotheses range from simple statements, such as moving a button to increase conversion, to robust ones involving multiple assumptions about user behavior to improve product adoption and retention. Ultimately, working with design hypotheses empowers designers to actively shape product development and enhances their professional growth by integrating them into a proactive role beyond mere execution.