This document introduces the concept of a Marketing Data Lake as a new way for marketing departments to be more data-driven. It argues that traditional marketing has been broken for not being able to prove what works and connect activities to outcomes. The authors describe how they built a Marketing Data Lake at their company Informatica in 60 days, which transformed marketing by allowing them to see the end-to-end customer journey and which activities drove revenue. The book aims to help other marketers fix their systems by building their own Data Lakes to make truly data-driven, accountable, and customer-focused marketing decisions.