  (Abridged) Cosmology faces three distinct challenges in the next decade. (1)
The dark sector, both dark matter and dark energy, dominates the Universe. Key
questions include determining the nature of both. Improved observational probes
are crucial. (2) Galaxy formation was initiated at around the epoch of
reionization: we need to understand how and when as well as to develop probes
of earlier epochs. (3) Our simple dark matter-driven picture of galaxy assembly
is seemingly at odds with several observational results, including the presence
of ULIRGS at high z, the `downsizing' signature, chemical signatures of
alpha-element ratios and suggestions that merging may not be important in
defining the Hubble sequence. Understanding the physical implications is a
major challenge for theorists and refiniing the observational uncertainties a
major goal for observers.
