NASA used to run a Metrics Data Program (MDP) to analyze data from software projects. In 2003, the research lead, Kenneth McGill, asked: “What can you learn from all that data?” McGill's challenge (and funding support) resulted in much work. The MDP is no more but its data was the seed for the PROMISE repository (Figure 1.1). At the time of this writing (2014), that repository is the focal point for many researchers exploring data science and software engineering. The authors of this book are long-time members of the PROMISE community.