  Active galactic nuclei come in many varieties. A minority of them are
radio-loud, and exhibit two opposite prominent plasma jets extending from the
proximity of the supermassive black hole up to megaparsec distances. When one
of the relativistic jets is oriented closely to the line of sight, its emission
is Doppler beamed and these objects show extreme variability properties at all
wavelengths. These are called "blazars". The unpredictable blazar variability,
occurring on a continuous range of time-scales, from minutes to years, is most
effectively investigated in a multi-wavelength context. Ground-based and space
observations together contribute to give us a comprehensive picture of the
blazar emission properties from the radio to the gamma-ray band. Moreover, in
recent years, a lot of effort has been devoted to the observation and analysis
of the blazar polarimetric radio and optical behaviour, showing strong
variability of both the polarisation degree and angle. The Whole Earth Blazar
Telescope (WEBT) Collaboration, involving many tens of astronomers all around
the globe, has been monitoring several blazars since 1997. The results of the
corresponding data analysis have contributed to the understanding of the blazar
phenomenon, particularly stressing the viability of a geometrical
interpretation of the blazar variability. We review here the most significant
polarimetric results achieved in the WEBT studies.
