Traditional requirements engineering for single systems, while remaining a large challenge for engineers, has been extensively researched and many techniques have been proposed and used with varying degree of success. However, many modern systems of systems are being developed to support interaction across multiple controlling authorities and existing techniques are proving to be inadequate for meeting the challenges of requirements engineering for systems of systems. This paper discusses some of these challenges, examines several existing techniques, and discusses how these techniques could be applied to engineer requirements for systems of systems.