Opportunistic systems are an emerging networking paradigm where communication between the source and destination occurs on-the-fly and depends on the availability of communication resources. Opportunistic systems also face significant design challenges, including finite battery energy, reliability due to capacity issues as well as intermittent communication, which act as barriers to efficient communication among users. In addition, novel techniques aim to extend the survivability of such systems in terms of both performance and reliability. Motivated by further examining recent advances in this field and promoting the optimization of the existing methodologies and/or approaches, this special issue aims to penetrate deeper into this field, in order to promote novel approaches in analyzing, designing and optimizing large-scale mobile opportunistic systems by exploring new and innovative schemes in order to improve the overall performance of opportunistic networks. Additionally, it aims to present novel resource management architectures and techniques for optimizing the connectivity and reliability aspects, as well as aims to propose new models for capacity and data flow management for providing sufficient end-to-end resource management. Moreover, it provides sufficient state-of-the-art research and current trends in opportunistic networking, combining the mobile communication, mobility and resource management field. It also scientifically presents the various concepts that contribute to enable high performance computing in these environments. Eight papers were selected out of 43 submitted, which were reviewed by qualified anonymous referees according to the practices of this journal. These papers cover a variety of important and challenging topics in the area of opportunistic systems and address various aspects of infrastructures and performance. Azzaz and Saidane introduced three proactive maintenance strategies for static wireless sensor networks for opportunistic communications using a limited number of mobile maintainer bots. The proposed maintenance strategies are based on a simple energy dissipation analytical model to estimate the occurrence times of the expected sensor failures in the network. Bourdena et al. discuss an efficient administration of radio spectrum resources, by exploiting a novel Radio Resource Management (RRM) framework, adopted in a spectrum broker, which is in charge to effectively orchestrate the available wireless networking resources. Goleva et al. investigate the buffer queueing behavior in high-speed opportunistic networks. They show statistical analyses of several types of traffic sources in a 3G network for which several measurements were collected in order to understanding of the statistical characteristics of the sources and, on the other hand, enable forecasting traffic behavior in the network in opportunistic systems. Dias, Rodrigues, and Zhou discuss cooperation approaches that are used to force nodes to share their own resources. Authors propose four different cooperation strategies for vehicular delay-tolerant networks and study their impact on the performance of the network. Nomikos et al. propose a buffer-aided successive opportunistic relaying scheme that aims at improving the average capacity of the network when inter-relay interference arises between relays that are selected for simultaneous transmission and reception. Tung et al. propose a disruption-tolerant network-based system called MetroNet, which exploits the precise schedules of metro systems to pre-fetch data and facilitate mobile Internet downloads, even if an end-to-end path between the source (i.e. content sources) and the destination (i.e. users) does not exist contemporaneously. Ciobanu et al. investigate the mobile interaction trace collected for a specific geolocation and analyse it in terms of the predictability of encounters and contact durations. Authors show that there is a regular pattern in the contact history of a node and efficiently predict the number of contacts per time unit in the future. Xiao et al. investigate how to preserve the limited energy of secondary units (SUs) in mobile cognitive networks by proposing a new sleeping action with which mobile SUs can better exploit the trade-off between their consumption and data transmission throughput. We would like to thank all authors who submitted papers, including those whose papers were not selected for this special issue. A special note of thanks goes to all of the referees for donating their abundant time and effort. Without them, this issue would not be possible. We hope that contributions in this special issue will stimulate further research in the important area of mobile opportunistic systems.