  Phase retrieval is to recover the signals from phaseless measurements which
is raised in many areas. A fundamental problem in phase retrieval is to
determine the minimal measurement number $m$ so that one can recover
$d$-dimensional signals from $m$ phaseless measurements. This problem attracts
much attention of experts from different areas. In this paper, we review the
recent development on the minimal measurement number and also raise many
interesting open questions.
