Forecasting is the process of making statements about events whose actual outcomes have not yet been observed. A more general term of forecasting is prediction. Prediction plays a very important role in our daily life. Its scope is extremely wide: weather forecasting, stock market prediction, supply-demand prediction, population forecasting, traffic prediction, protein-protein interaction prediction, etc. In order to facilitate our discussion, we can only include some topics that are widely being discussed and can reflect the interest of most people. Specifically, this book presents how forecasting models are being used and why they are important.