This document outlines 15 lessons learned from building large-scale machine learning systems in the real world. Some key challenges discussed include data scientists not being well-suited for engineering work, traditional development methodologies not working for machine learning, the difficulty of data labeling and feature extraction, and the complexities of training, executing, operationalizing, and securing machine learning models at scale. The document provides ideas to address these challenges such as establishing separate data science and engineering teams, implementing automated data labeling strategies, leveraging centralized feature stores, and adopting techniques like transfer learning and continual learning.