This document discusses lean prototyping for the lean UX process. It defines different types of prototypes based on fidelity, interactivity, purpose, and whether they represent behavior or structure. It compares prototypes to other deliverables like sketches, wireframes, storyboards, and mockups. The document outlines the lean UX process and how it differs from waterfall. It provides examples of prototyping tools and discusses best practices for lean prototyping like focusing on one-bite projects and iterating quickly based on user feedback.