  Observations from the two STEREO-spacecraft give us for the first time the
possibility to use stereoscopic methods to reconstruct the 3D solar corona.
Classical stereoscopy works best for solid objects with clear edges.
Consequently an application of classical stereoscopic methods to the faint
structures visible in the optically thin coronal plasma is by no means straight
forward and several problems have to be treated adequately: 1.)First there is
the problem of identifying one dimensional structures -e.g. active region
coronal loops or polar plumes- from the two individual EUV-images observed with
STEREO/EUVI. 2.) As a next step one has the association problem to find
corresponding structures in both images. 3.) Within the reconstruction problem
stereoscopic methods are used to compute the 3D-geometry of the identified
structures. Without any prior assumptions, e.g., regarding the footpoints of
coronal loops, the reconstruction problem has not one unique solution. 4.) One
has to estimate the reconstruction error or accuracy of the reconstructed
3D-structure, which depends on the accuracy of the identified structures in 2D,
the separation angle between the spacecraft, but also on the location, e.g.,
for east-west directed coronal loops the reconstruction error is highest close
to the loop top. 5.) Eventually we are not only interested in the 3D-geometry
of loops or plumes, but also in physical parameters like density, temperature,
plasma flow, magnetic field strength etc. Helpful for treating some of these
problems are coronal magnetic field models extrapolated from photospheric
measurements, because observed EUV-loops outline the magnetic field. This
feature has been used for a new method dubbed 'magnetic stereoscopy'. As
examples we show recent application to active region loops.
