This talk will describe the development and electrical characterization of two classes of molecular electronic components. The first class of structures involves metal-molecule-metal systems with pre-formed metal contacts, primarily lateral break junctions formed either by electromigration or by shadow evaporation. A number of molecular species have been studied using these structures, including short aromatic thiols and short DNA double strands with thiol bonding groups at each end. The electrical characteristics of these devices indicate that strong coupling between the contacts and the molecular species can be realized. The second class of devices involves metal/molecule/semiconductor device structures, which are lithographically defined and fabricated using an indirect evaporation technique for the metal (top) contact and p+ GaAs for the bottom contact. In these structures, the electronic conduction between the metal and semiconductor can be modulated by choice of molecular species. Several alkyl thiol and aromatic thiol molecules have been employed in order to determine the effects of molecular length, conjugation and intrinsic dipole moment. The current-voltage characteristics and conductance versus temperature both indicate that the molecular layers change the transport mechanism, generally involving a lower effective barrier height than that of a metal/semiconductor Schottky barrier. These results reflect previous studies in which nanoscale metal/molecule/semiconductor structures exhibited low resistance contacts, implying that effective coupling and control of the surface electrical properties can be achieved using a molecular layer.@footnote 1@ A simple model for the conduction has been developed, utilizing our prior studies on surface Fermi level unpinning in GaAs structures.@footnote 2@ @FootnoteText@ @footnote 1@ T. Lee, et al., APL 76, 212 (2000). @footnote 2@ S. Lodha, et al., Appl. Phys. Lett. 80, 4452 (2002).