Delay- and Disruption Tolerant Networks (DTNs) are networks subject to arbitrarily long-lived disruptions in connectivity and therefore cannot guarantee end-to-end connectivity at all times. Consequently DTNs called for novel core networking protocols since most existing Internet protocols rely on the network's ability to maintain end-to-end communication between participating nodes. This book presents the fundamental principles that underline DTNs. It explains the state-of-the-art on DTNs, their architecture, protocols, and applications. It also explores DTN's future technological trends and applications. Its main goal is to serve as a reference for researchers and practitioners.