Large software-intensive systems have emerged in several application domains, such as healthcare, transportation, smart environments, and Industry 4.0. Sometimes referred to as Systems-of-Systems (SoS) and resulted from the integration of various heterogeneous and independent software systems, such large systems lead to high dynamism and often operate in critical and uncertain environments. Hence, the reliability of these systems must be a concern, but traditional reliability approaches fail to cope with such high dynamism and uncertainty. Thus, new solutions are required to ensure the reliability of SoS. Moreover, there is a lack of studies that investigate the reliability of SoS. This paper presents the state of the art of the way that the reliability of SoS has been addressed. After investigating the literature, we selected 27 studies to perform a detailed analysis regarding factors that affect the reliability of SoS and approaches and metrics to improve it. We found an area still gaining maturity with researchers working in isolation and mainly developing solutions for domain-specific problems. There are still various critical open issues, while SoS have been increasingly adopted as a suitable and integrated solution in critical domains.