Self-assembled monolayers (SAMs) of alkanethiols and disulfides on gold (see Figure) form organic interfaces with properties largely controlled by the end groups of the molecules composing the film. Scanning tunneling microscopy (STM) at picoamperes proves to be a powerful technique for probing the surface structure of such SAMs down to the sub-angstrom level. The application of STM to these systems and the structure-property information obtained is reviewed.