Simply put, hypothesis-driven development (HDD) is the process of stating the desired impact of a new product feature, and then measuring the actual impact of that feature on real users, often using online controlled experiments, commonly called A/B tests. In a world where the opinions of even expert product and UX people is wrong as often as it is right, the HDD approach to product development is important for consumer-facing businesses of any size. But we'll make the case that as a consumer-facing online business scales to thousands of daily-active-users, HDD combined with extensive A/B testing becomes absolutely crucial in growing a business to the next level.