Index coding is a canonical problem in network information theory that studies the fundamental limit and optimal coding schemes for broadcasting multiple messages to receivers with different side information. The index coding problem provides a simple yet rich model for several important engineering tasks such as satellite communication, content broadcasting, distributed caching, device-to-device relaying, and interference management. This monograph aims to provide a broad overview of this fascinating subject, focusing on the simplest form of multiple-unicast index coding. A unified treatment on coding schemes based on graph-theoretic, algebraic, and information-theoretic approaches is presented. Although the problem of characterizing the optimal communication rate is open in general, several bounds and structural properties are established. The relationship to other problems such as network coding and distributed storage is also discussed.