  CMOS Monolithic Active Pixel Sensors (CPS) are ultra-light and highly
granular silicon pixel detectors suited for highly sensitive charged particle
tracking. Being manufactured with cost efficient standard CMOS processes, CPS
may integrate sensing elements together with analogue and digital data
processing circuits into one monolithic chip. This turns into $50~\rm \mu m$
thin sensors, which provide an outstanding typical spatial resolution of few
$~\rm \mu m$ and a detection efficiency for minimum ionizing particles above
$99.9\%$. The radiation tolerance of CPS was initially constrained by the
limits of the CMOS processes used for their production but has been improved by
orders of magnitudes during the last years.
  This work reviews the related R&D on the radiation tolerance of traditional
CPS with partially depleted active medium as pioneered by the MIMOSA-series
developed by the IPHC Strasbourg. Procedures for assessing radiation damage in
those non-standard pixels are discussed and the major mechanisms of radiation
damage are introduced. Techniques for radiation hardening are shown. Moreover,
recent results on next generation CPS featuring fully depleted sensors based on
improved CMOS processes are summarized.
