System of Systems (SoS) Engineering deals with the planning, analyzing, organizing, and integrating multiple system capabilities into existing and new systems which will provide more capability. It may deliver capabilities by combining multiple collaborative and autonomous-yet-interacting systems. The mix of systems may include existing, partially developed, and yet-to-be-designed independent systems. The goal is to pool these resources and capabilities together to create a new, more complex system that offers more functionality and performance than simply the sum of the constituent systems. That’s why a SoS is defined as a set or arrangement of systems that results when independent and useful systems are integrated into a larger system that delivers unique capabilities.
The DoD is increasingly moving from a platform focus to an emphasis on SoS. DoD SoS are not new-start acquisitions; they are modifications to ensembles of existing systems that together address needs. An SoS is an overlay on systems, wherein the systems retain their identity, and management and engineering continue concurrently with the SoS. Rather than controlling systems, SoS systems engineers collaborate with the systems engineers of constituent systems. 
Guide: Summary Systems Engineering Guide for Systems of Systems (SoS)
Guide: DoD Systems Engineering Guide for Systems of Systems – Aug 2008
There are seven (7) core elements that characterize systems engineering for SoS. They are: 
- Translating SoS capability objectives into high-level SoS requirements
- Understanding the constituent systems and their relationships
- Assessing the extent to which SoS performance meets capability objectives
- Developing, evolving, and maintaining an architecture for the SoS
- Monitoring and assessing potential impacts of changes on SoS performance
- Addressing SoS requirements and solution options
- Orchestrating upgrades to SoS
There are four (4) types of SoS that are commonly found in the DoD today. These are: [1,2]
- Virtual: Virtual SoS lack a central management authority and a centrally agreed-upon purpose for the system-of-systems.
- Collaborative: In collaborative SoS the component systems interact more or less voluntarily to fulfill agreed-upon central purposes. The Internet is a collaborative system. The Internet Engineering Task Force works out standards but has no power to enforce them. The central players collectively decide how to provide or deny service, thereby providing some means of enforcing and maintaining standards.
- Acknowledged: Acknowledged SoS have recognized objectives, a designated manager, and resources for the SoS; however, the constituent systems retain their independent ownership, objectives, funding, and development and sustainment approach. Changes in the systems are based on collaboration between the SoS and the system.
- Directed: Directed SoS are those in which the integrated system-of-systems is built and managed to fulfill specific purposes. It is centrally managed during long-term operation to continue to fulfill those purposes as well as any new ones the system owners might wish to address. The component systems maintain an ability to operate independently, but their normal operational mode is subordinated to the centrally managed purpose.
AcqLinks and References:
-  Defense Acquisition Guidebook (DAG)
-  Systems Engineering Guide for Systems of Systems (SoS) – Aug 2008
-  Summary – Systems Engineering Guide for Systems of Systems (SoS) – Dec 2010
- Website: OSD Systems Engineering – SoS Systems Engineering
- Presentation: Key System of Systems Engineering Artifacts to Guide Engineering Activities; Dr. Jo Ann Lane, USC