  The physical process leading to the acceleration of the expansion of the
universe is unknown. It may involve new high energy physics or extensions to
gravitation. Calling this generically dark energy, we examine the consistencies
and relations between these two approaches, showing that an effective equation
of state function w(z) is broadly useful in describing the properties of the
dark energy. A variety of cosmological observations can provide important
information on the dynamics of dark energy and the future looks bright for
constraining dark energy, though both the measurements and the interpretation
will be challenging. We also discuss a more direct relation between the
spacetime geometry and acceleration, via ``geometric dark energy'' from the
Ricci scalar, and superacceleration or phantom energy where the fate of the
universe may be more gentle than the Big Rip.
