  Magnetic fields of cool stars can be directly investigated through the study
of the Zeeman effect on photospheric spectral lines using several approaches.
With spectroscopic measurement in unpolarised light, the total magnetic flux
averaged over the stellar disc can be derived but very little information on
the field geometry is available. Spectropolarimetry provides a complementary
information on the large-scale component of the magnetic topology. With
Zeeman-Doppler Imaging (ZDI), this information can be retrieved to produce a
map of the vector magnetic field at the surface of the star, and in particular
to assess the relative importance of the poloidal and toroidal components as
well as the degree of axisymmetry of the field distribution. The development of
high-performance spectropolarimeters associated with multi-lines techniques and
ZDI allows us to explore magnetic topologies throughout the Hertzsprung-Russel
diagram, on stars spanning a wide range of mass, age and rotation period. These
observations bring novel constraints on magnetic field generation by dynamo
effect in cool stars. In particular, the study of solar twins brings new
insight on the impact of rotation on the solar dynamo, whereas the detection of
strong and stable dipolar magnetic fields on fully convective stars questions
the precise role of the tachocline in this process.
