  We discuss the factors that determine the overall shape and magnitude of the
current-voltage (I-V) characteristics of a variety of molecular conductors
sandwiched between two metallic contacts. We analyze the individual influences
of the contact geometry, the molecular chemistry, the electrostatics of the
environment, and charging on molecular conduction. Current conduction depends
sensitively on the experimental geometry, as well as on the theoretical model
for the molecule and the contacts. Computing molecular I-V characteristics will
thus require theoretical understanding on several fronts, in particular, (a) in
the scheme for calculating the molecular energy levels, as well as (b) on the
position of the contact Fermi energy relative to those levels.
