Real time control by multiwavelength phase modulated ellipsometry (PME), as applied to the growth of optical coatings deposited on various substrates, is reviewed. The structures consist of plasma deposited SiO2, and SiNx, stacks and SiOxNy gradient-index coatings. A feedback method is used for growth control of a standard quarterwave filter on c-Si and on glass, the latter achieved by incorporating incoherent modelling of the transparent substrate. The feedback control method is based on a comparison between the real time PME measurements and pre-computed target trajectories. The resulting optical coatings characterised by spectroscopic PME and transmission measurements show reproducible spectral characteristics, with less than 1% deviation between the target and measured spectral responses. The feedback control method is further generalised to the control of gradient-index coatings.