Opportunistic networks forms one of the two major categories of multi-hop ad hoc networks, which are widely used in many scenarios. Applications on opportunistic networks have been on mobile network, space networks, online social networks, and underwater networks, etc. This book introduces opportunistic networking by combining fundamental mathematical models, system architecture/protocols, and practical applications. The book summarizes the main outcomes and investigation of opportunistic networks and provides the overview picture and system training for newcomers to this promising area.