The special issue of Aerospace Science and Technology has topics related to instability and control of massively separated flows. This volume contains work performed during the execution of an EU FP7 Marie Curie International Research Scientists Exchange Scheme (IRSES) grant in the period of January 1, 2011 to December 31, 2014. The Instability and Control of Massively Separated Flows (ICOMASEF) IRSES Grant has been coordinated by the author and organized around the joint research interests of the groups led by several researchers from a number of universities.