  High-Dimensional Dynamic Factor Models are presented in detail: The main
assumptions and their motivation, main results, illustrations by means of
elementary examples. In particular, the role of singular ARMA models in the
theory and applications of High-Dimensional Dynamic Factor Models is
discussed.The emphasis of the paper is on model classes and their structure
theory, rather than on estimation in the narrow sense. Our aim is not a
comprehensive survey. Rather we try to point out promising lines of research
and applications that have not yet been sufficiently developed.
