  Deep imaging is a fundamental tool in the study of the outermost structures
of galaxies. We review recent developments in ultra-deep imaging of galaxy
disks and haloes, highlighting the technical advances as well as the
challenges, and summarizing observational results in the context of modern
theory and simulations. The deepest modern galaxy imaging comes from three main
sources: (1) surveys such as the Sloan Digital Sky Survey's Stripe 82 project,
(2) very long exposures on small telescopes, including by amateurs, and (3)
long exposures on the largest professional telescopes. The technical challenges
faced are common in all these approaches, and include the treatment of light
scattered by atmosphere and telescope/instrument, correct flat fielding, and
the subtraction of non-galaxy light in the images. We review scientific results
on galaxy disks and haloes obtained with deep imaging, including the detection
and characterization of stellar haloes, tidal features and stellar streams,
disk truncations, and thick disks. The area of ultra-deep imaging is still very
much unexplored territory, and future work in this area promises significant
advances in our understanding of galaxy formation and evolution.
