  Software-intensive Systems-of-Systems (SoS) refer to an arrangement of
managerially and operationally independent systems(i.e., constituent systems),
which work collaboratively towards the achievement of global missions. Because
some SoS are developed for critical domains, such as healthcare and
transportation, there is an increasing need to attain higher quality levels,
which often justifies additional costs that can be incurred by adopting formal
and semi-formal approaches (i.e., languages and techniques) for modeling
requirements. Various approaches have been employed, but a detailed landscape
is still missing, and it is not well known whether they are appropriate for
addressing the inherent characteristics of SoS. The main contribution of this
article is to present this landscape by reporting on the state of the art in
SoS requirements modeling. This landscape was built by means of a systematic
mapping and shows formal and semi-formal approaches grouped from model-based to
property-oriented ones. Most of them have been tested in safety-critical
domains, where formal approaches such as finite state machines are aimed at
critical system parts, while semi-formal approaches (e.g., UML and i*) address
non-critical parts. Although formal and semi-formal modeling is an essential
activity, the quality of SoS requirements does not rely solely on which
formalism is used, but also on the availability of supporting tools/mechanisms
that enable, for instance, requirements verification along the SoS lifecycle
