Thin films of TiO2 were prepared by reactive d.c. sputtering using different magnetrons and for comparison by r.f. diode sputtering and reactive evaporation. Most of the films were annealed after deposition. Amorphous, anatase, or anatase/rutile films can be obtained depending on the deposition and annealing parameters. From these experiments and from literature results we conclude that substrate temperature and the energy of the particles impinging on the substrate but not directly the oxygen partial pressure are the relevant parameters which determine the film structure. Annealing experiments show that below 600-degrees-C anatase grows faster than rutile. The nucleation of rutile is favoured by increasing process energy and by the presence of Ti in the vapour phase. The nucleation and growth behaviour of TiO2 in an amorphous matrix can be understood from the differences in packing density of the involved phases. The influence of the process parameters on the optical properties of TiO2 films, such as index of refraction, scattering of light, and absorption, is discussed.