The last decade has seen explosive growth in the both the materials and processing of organic semiconductors. Relatively ignored, is the interface (or surface) of the organic semiconductor though the surface properties are central to electronic band structure, stiction, corrosion/water resistance, and (quenching) trap states (to name but a few). We have recently developed the ability to chemically functionalize the surface of organic semiconductors to form a monolayer not dissimilar to those found on other classic substrates. Specifically, we can tailor the chemical functionalities at the surface of the acene class of organic semiconductors using the Diels-Alder reaction, where the molecule of interest is dosed in via the vapor phase and bonds exclusively at the surface. Functionalities demonstrated as appendable include cyano groups, anhydrides, maleimides, halides, and alkyl chains, with many more possible. This talk outlines the fundamental properties of these monolayers (density, the nature of the bonding, kinetics of formation) and the ability to influence materials properties such as conductivity and stability, and touches on some of the more unique aspects of this surface chemistry including the high degree of face selectivity intrinsic to the reaction.