Rough volatility connects the microstructure of financial markets with the large-scale behavior of volatility. This approach results in parsimonious models that are consistent across different time-scales and show remarkable agreement with econometric data. In this first lecture of two, we will focus on the econometric motivation of rough volatility and explore its consequences for pricing, focusing on the rough Bergomi model. As an application, we will show how to forecast realized variance and the forward variance curve.