This document introduces the concept of customer development and summarizes its key phases over the past decade. It begins with describing how traditional product development models made incorrect assumptions about knowing customer problems and product solutions. It then outlines Steve Blank's customer development model, including the phases of customer discovery, customer validation, customer creation, and company building. It emphasizes the importance of getting outside the building to test hypotheses through customer feedback in early phases via continuous discovery and "do-over" iterations until a repeatable and scalable business model is proven.