The papers in this special issue on security and dependability of cloud systems and services. Service-based cloud computing systems are used nowadays in many business- and mission-critical scenarios. As the service-oriented paradigm increasingly spreads in a wide range of application fields, including big data, cloud storage, mobile cloud computing, and sensor cloud, there is a growing need for sound methodologies, algorithms and techniques for building services in which companies, organizations and citizens can trust and rely upon. Security and dependability are therefore becoming more and more relevant concerns for such systems, whose complexity, heterogeneity, and fast-changing dynamics bring difficult challenges to the research and industry communities.