  With multiple vantage points around the Sun, STEREO and SDO imaging
observations provide a unique opportunity to view the solar surface
continuously. We use He II 304 A data from these observatories to isolate and
track ten active regions and study their long-term evolution. We find that
active regions typically follow a standard pattern of emergence over several
days followed by a slower decay that is proportional in time to the peak
intensity in the region. Since STEREO does not make direct observations of the
magnetic field, we employ a flux-luminosity relationship to infer the total
unsigned magnetic flux evolution. To investigate this magnetic flux decay over
several rotations we use a surface flux transport model, the Advective Flux
Transport (AFT) model, that simulates convective flows using a time-varying
velocity field and find that the model provides realistic predictions when
information about the active region's magnetic field strength and distribution
at peak flux is available. Finally, we illustrate how 304 \AA\ images can be
used as a proxy for magnetic flux measurements when magnetic field data is not
accessible.
