  Observational astrophysics started when spectroscopy could be applied to
astronomy. Similarly, observational work on stellar magnetic fields became
possible with the application of spectro-polarimetry. In recent decades there
have been dramatic advances in the observational tools for spectro-polarimetry.
The four Stokes parameters that provide a complete representation of partially
polarized light can now be simultaneously imaged with megapixel array detectors
with high polarimetric precision (0.001% in the degree of polarization). This
has led to new insights about the nature and properties of the magnetic field,
and has helped pave the way for the use of the Hanle effect as a diagnostic
tool beside the Zeeman effect. The magnetic structuring continues on scales
orders of magnitudes smaller than the resolved ones, but various types of
spectro-polarimetric signatures can be identified, which let us determine the
field strengths and angular distributions of the field vectors in the spatially
unresolved domain. Here we review the observational properties of the magnetic
field, from the global patterns to the smallest scales at the magnetic
diffusion limit, and relate them to the global and local dynamos.
