This document provides a practical guide to testing in direct marketing. It discusses why testing is important, key concepts like sampling distributions and statistics, and outlines a structured testing lifecycle approach. This includes developing an annual testing strategy to prioritize questions, designing individual tests by calculating appropriate sample sizes and coding treatments, executing the test, evaluating results using statistical analysis to determine significance, and building insights to update strategy. Cautionary tales highlight the importance of testing value, sample size, evaluating multiple variables, and setting appropriate expectations. The overall goal is to make evidence-based marketing decisions through incremental, well-planned testing.