Methods that can be used to tailor the surface properties of semiconductors will become increasingly important as new applications for semiconductor-based materials continue to be developed. The attachment of organic groups in particular can impart new functionality to the surface, providing properties such as passivation, molecular recognition, lubrication, or biocompatibility. This article will focus on organic functionalization of Group IV surfaces using vapor phase delivery in a dry processing environment. A combination of experimental and theoretical methods has been applied to identify the bonding and reactivity of the organic layers at the semiconductor surface. The attachment chemistry of dienes and amines at the Si(100)-2 x 1 surface will be described. We show that the [4+2] cycloaddition (Diels-Alder reaction) occurs readily for a range of conjugated dienes at the (100)-2 x I surface of Si, and that the reaction occurs at the surfaces or Ge(100) and C(100) as well. In amine reactivity, competing reaction pathways such as N-H bond dissociation and dative bonding through the nitrogen lone pair have been observed. The potential for these different classes of attachment reactions to impact future applications will be discussed.