This document outlines lessons learned from 100,000 A/B tests. The key lessons are: 1) Define specific success metrics upfront; 2) Reduce choices in experiments to improve conversion rates; 3) Carefully craft wording of calls to action as small changes can significantly impact clicks and purchases; 4) Fail experiments quickly if they are not working to avoid wasted time; and 5) Consider how changes could impact the overall user experience and metrics, not just a single factor. Examples of successful A/B tests that illustrate each lesson are also provided.