Systems-of-Systems (SoS) have become increasingly complex and are frequently used in highly distributed, dynamic, and open environments. SoS have emerged as a new class of evolving software systems, where constituent systems (themselves systems in their own right) work cooperatively in order to fulfil specific missions. SoS are characterised by operational and managerial independence, distribution, evolutionary development, and creation of emergent behaviour. These characteristics have therefore raised substantial challenges to software engineering researchers and practitioners. SoS and especially Software-intensive SoS (SiSoS) is becoming a hotspot, from both the research and the industrial viewpoints, in particular, related to crucial classes of applications. Furthermore, SiSoS are increasingly involved in applications that address societal needs, such as environmental monitoring, distributed energy grids management, emergency coordination and crisis management, global traffic control, and smart cities. In this context, the research topic of software engineering for SoS and SiSoS needs to receive more attention and research effort, considering that the software part of SoS is becoming increasingly larger and more complex. Software engineering theories and tools are needed to adequately design, implement, operate, and even evolve such dynamic systems.