  The physics aims at the proposed future CLIC high-energy linear $e^+ e^-$
collider pose challenging demands on the performance of the detector system. In
particular the vertex and tracking detectors have to combine precision
measurements with robustness against the expected high rates of beam-induced
backgrounds. A spatial resolution of a few microns and a material budget down
to 0.2\% of a radiation length per vertex-detector layer have to be achieved
together with a few nanoseconds time stamping accuracy. These requirements are
addressed with innovative technologies in an ambitious detector R\&D programme,
comprising hardware developments as well as detailed device and Monte Carlo
simulations based on TCAD, Geant4 and Allpix-Squared. Various fine pitch hybrid
silicon pixel detector technologies are under investigation for the CLIC vertex
detector. The CLICpix and CLICpix2 readout ASICs with \SI{25}{\micro\meter}
pixel pitch have been produced in a \SI{65}{\nano\meter} commercial CMOS
process and bump-bonded to planar active edge sensors as well as capacitively
coupled to High-Voltage (HV) CMOS sensors. Monolithic silicon tracking
detectors are foreseen for the large surface ($\approx$
\SI{140}{\meter\squared}) CLIC tracker. Fully monolithic prototypes are
currently under development in High-Resistivity (HR) CMOS, HV-CMOS and Silicon
on Insulator (SOI) technologies. The laboratory and beam tests of all recent
prototypes profit from the development of the CaRIBou universal readout system.
This talk presents an overview of the CLIC pixel-detector R\&D programme,
focusing on recent test-beam and simulation results.
