  This talk presents a pedagogical discussion of how precision
distance-redshift observations can map out the recent expansion history of the
universe, including the present acceleration and the transition to matter
dominated deceleration. The proposed Supernova/Acceleration Probe (SNAP) will
carry out observations determining the components and equations of state of the
energy density, providing insights into the cosmological model, the nature of
the accelerating dark energy, and potentially clues to fundamental high energy
physics theories and gravitation. This includes the ability to distinguish
between various dynamical scalar field models for the dark energy, as well as
higher dimension and alternate gravity theories. A new, advantageous
parametrization for the study of dark energy to high redshift is also
presented.
