This document discusses the concept of product-market fit and how startups can achieve it. It defines product-market fit as when a startup finds a repeatable and scalable business model. It recommends following the customer development process and lean startup methodology to rapidly test hypotheses through minimum viable products and pivot if needed. Case studies of Foursquare are presented, showing how it achieved early product-market fit by responding to customers but now faces challenges in scaling, monetization, and competitive response.