We will review theoretical work on electron transport through molecules, as it is being applied to molecular electronic systems. There are several issues that are relevant for a quantitative description of the current voltage characteristics of even a simple molecular electronics system. Many of the basic concepts are understood, but general quantitative agreement with experiment is still rarely achieved. Some specific issues that will be reviewed are: the alignment of the metal’s Fermi level within the molecular HOMO-LUMO gap, the effect of a gate potential, and the dependence of the current on the length of the molecule. We discuss a complex bandstructure approach, which gives a simple framework to understand the length dependence of the current (the exponential (beta factor) for some model molecules composed of fragments of infinite chains (such as n-alkanes). This allows simple estimates of the current to be obtained with little calculation. This framework also produces an interesting view of the tunneling time through the molecule.