  We present the design and performance of RoboPol, a four-channel optical
polarimeter operating at the Skinakas Observatory in Crete, Greece. RoboPol is
capable of measuring both relative linear Stokes parameters $q$ and $u$ (and
the total intensity $I$) in one sky exposure. Though primarily used to measure
the polarization of point sources in the R-band, the instrument features
additional filters (B, V and I), enabling multi-wavelength imaging polarimetry
over a large field of view (13.6' $\times$ 13.6'). We demonstrate the accuracy
and stability of the instrument throughout its five years of operation. Best
performance is achieved within the central region of the field of view and in
the R band. For such measurements the systematic uncertainty is below 0.1% in
fractional linear polarization, $p$ (0.05% maximum likelihood). Throughout all
observing seasons the instrumental polarization varies within 0.1% in $p$ and
within 1$^\circ$ in polarization angle.
