Cognitive load theory, originally developed for educational psychology by John Sweller, has profound implications for interface design. The theory distinguishes three types of cognitive load: intrinsic (complexity of the task itself), extraneous (caused by poor design), and germane (productive effort that builds understanding). This article translates these concepts into practical design principles: chunking information, progressive disclosure, consistent patterns, and reducing extraneous load through clear visual hierarchy. It includes before-and-after examples of interfaces redesigned using cognitive load principles with measurable usability improvements.