The burgeoning fields of organic electronics, molecular electronics, and bio-electronics are all placing increased emphasis on electrically interfacing inorganic materials such as silicon and diamond with organic and biological materials. Over the last several years we have been exploring new methods for fabricating and patterning hybrid interfaces on silicon and diamond surfaces, and understanding how the specific chemical chemical bonds and nanometer-scale structures control the resulting electrical properties of the interfaces. In ultrahigh vacuum the reactions of C=C bonds, S-H bonds, and O-H bonds can be used as the basis for preparing covalently-bonded layers that exhibit specific reactive and/or non-reactive functional groups, which can in turn be used for linking more complex films (pentacene) and structures (DNA) to the surfaces . Essentially identical reactions are used to link organic molecules with H-terminated silicon and diamond under ambient (atmospheric pressure) conditions, using ultraviolet light to initiate the reactions. We have been assessing the molecular conductivity on an atomic level via STM measurements of the apparent height, and on more macroscopic scales by measuring interfacial impedance after the molecular monolayers are covered with conductive materials such as the pentacene or after they are linked to biological molecules such as DNA. In this talk I will summarize what we have learned about how to use organic monolayers as electrical interfaces to pentacene films, DNA molecules, and other nanometer-scale structures of interest.