Chapter 15 provides an introduction to Chaps. 16–19 in this book on how uncertainty is a core component of learning what limits our understanding of Earth’s climate system and its behaviors. Global Climate Models (aka Earth Systems Models) provide the tools that we use to investigate how the individual components work alone as well as for the fully coupled models. In this chapter, we discuss how uncertainties in climate models can be compared with observational data, as well as with other Earth System Models. At the core of these investigations, all model results are limited by the available computational resources. By prioritizing the available resources (e.g., people, models, data, scenarios, and computers) to generate useful results for different groups (IPCC, national governments, or local communities), the following chapters dive deeper into specific uncertainties associated with specific applications to assess uncertainty for key impacts and vulnerabilities.