The development of the unique morphology of ultrananocrystalline diamond (UNCD) films has been investigated by varying three important parameters which are expected to have a pronounced influence on the morphology and structure of any thin film, namely the deposition time, the growth rate, and the nucleation density. The films have been characterized, among others, by scanning electron microscopy (SEM), atomic force microscopy (AFM), Fourier transform infrared spectroscopy (FTIR), nuclear reaction analysis and variable angle ellipsometry. It turned out, however, that the morphology and the rms roughness are almost independent of these parameters. Growth starts from individual nucleation sites created by the substrate pre-treatment. From these sites individual isolated semispherical nodules develop which finally coalesce to form a continuous film. Once coalescence has taken place, the films loose their “memory” of their nucleation history. Rather, morphology and roughness are solely determined by the rate of secondary nucleation, which is very high and distinguishes UNCD from poly- and nanocrystalline diamond films.