  Few-body systems offer a unique challenge to those interested in deep
inelastic scattering. Using the deuteron as the simplest and most easily
solvable nuclear system we review the main physics issues in the valence and
sea quark regimes. For the former the key issue is the understanding of Fermi
motion and binding corrections, and our ability to extract the ratio F_2n/F_2p
as x -> 1. The most recent analysis of deuteron data for F_2n/F_2p is
consistent with perturbative QCD, but inconsistent with the assumptions common
to all standard parton distribution fits. This should be taken into account in
calculating event rates at HERA --- especially at large x and Q^2. The
extraction of g_1n also presents an opportunity to refine the description of
nuclear effects in spin-dependent structure functions. In the region of small x
one must correct for shadowing and meson exchange current effects in order to
explore the flavor structure of sea, which is, in turn, deeply related to the
chiral structure of the nucleon.
