Epitaxial growth of GaN layers is a difficult task due to the lack of lattice and thermal expansion matching substrates to GaN. Usually sapphire or SiC and lately also Si (111) wafers are employed with a lattice mismatch of 16, 3.5 and -17 %, and with a thermal mismatch of -34, 25 and 54 %, respectively. Therefore deposition strategies need to be developed which help compensate the mismatch in order to obtain high quality GaN templates for device structures. In this paper first the chemistry of the GaN growth process will be discussed. Then the growth procedure used in MOVPE will be presented. It will be shown how growth parameters influence the characteristics of the layers and how in situ characterization techniques can be used for growth monitoring and optimization. At last methods will be introduced which show how modeling and emission corrected pyrometry can be employed to increase the reproducibilty of the growth process.