Spectral feature selection is a state-of-the-art technique based on spectral graph theory. This book provides a comprehensive introduction to spectra, including its theoretical foundations, connections to existing feature selection and extraction algorithms, and its applications in solving novel real-world problems in feature selection. It covers general feature selection and feature extraction concepts, the most popular existing algorithms, and recent research developments. The text also includes precise definitions, a number of illustrative figures, and plenty of examples, along with slides, source code, and data on a supporting website.