The initial stage during vapor deposition has been extensively studied in physical vapor deposition (PVD) processes, and nucleation theories have been successfully used to model island nucleation processes during PVD. Compared with the extensive research in PVD, there has been less work on understanding the initial stage in chemical vapor deposition (CVD) processes, despite the technological and commercial importance of CVD-based manufacturing systems. In this work we briefly review the nucleation theories developed for PVD processes and consider the validity of them for modeling the initial stage of CVD processes. One characteristic of CVD processes is the existence of an incubation time. Recent research indicates that the incubation time can be caused by the different reactivity of precursors nucleating on substrates and islands. We proposed process indices to evaluate the relative importance of sticking probabilities and desorption of adsorbates on the incubation time. The differing precursor reactivity between islands and substrates may also affect the island growth mode. This situation in CVD processes differs from that in PVD processes, for which current nucleation theories were developed, and therefore prevents the direct application of PVD nucleation theories to CVD processes. Therefore, to model CVD processes, a nucleation model is needed that is sensitive to the different reactivity of precursors to islands and substrates. © 2004 Elsevier B.V. All rights reserved.