Very few phenomena are unaffected by time, which in itself is enough to justify the importance of understanding what time series are. In this first part of the book, we’ll define time series and explore the particularities of working with them. We’ll also develop our very first forecasting models using naive methods. These will serve as baseline models, and we’ll reuse these techniques throughout the book. Finally, we’ll study a situation where forecasting is not possible, so that we identify and avoid falling into that trap.