We rely on measurements to make decisions, but uncertainties are always inherent in the results. Therefore, it is essential to understand, assess, and report these uncertainties. Probability theory plays a crucial role in both presenting meaningful measurement results (through frequentist data analysis) and in assigning or updating probabilities for potential values of the measured quantity (through Bayesian inference). In this chapter, we will clarify the meanings of key terms and outline the framework of the book.