This document provides advice and a framework for conducting customer interviews and problem validation experiments as part of the lean startup methodology. It emphasizes the importance of getting customer feedback early through interviews in order to learn quickly and avoid wasting months or years building the wrong product. It outlines an iterative process of planning customer interview experiments, conducting the interviews, analyzing results to improve customer and problem definitions, and planning new experiments. The mindset encouraged is being open to learning from customers and using their insights as the key to startup success.