  For true insight into the nature of dark energy, measurements of the
precision and accuracy of the Supernova/Acceleration Probe (SNAP) are required.
Precursor or scaled-down experiments are unavoidably limited, even for
distinguishing the cosmological constant. This white paper presents an overview
of the necessity for SNAP and the role of precursor experiments; accompanying
white papers detail the supernovae and weak lensing parts of the experiment.
