  The 3D magnetic topology of a solar active region (NOAA 10956) was
reconstructed using a linear force-free field extrapolation constrained using
the twin perspectives of \emph{STEREO}. A set of coronal field configurations
was initially generated from extrapolations of the photospheric magnetic field
observed by the Michelson Doppler Imager (MDI) on \emph{SOHO}. Using an EUV
intensity-based cost function, the extrapolated field lines that were most
consistent with 171\AA\ passband images from the Extreme UltraViolet Imager
(EUVI) on \emph{STEREO} were identified. This facilitated quantitative
constraints to be placed on the twist ($\alpha$) of the extrapolated field
lines, where $\nabla \times {\bf B} = \alpha {\bf B}$. Using the constrained
values of $\alpha$, the evolution in time of twist, connectivity, and magnetic
energy were then studied. A flux emergence event was found to result in
significant changes in the magnetic topology and total magnetic energy of the
region.
