Research into the light- and heat-induced yellowing of lignocellulose and cellulose pulps and related model systems is reviewed with reference to the work done by the author and coworkers during fifteen years of research. The mechanism of yellowing was investigated by performing photochemical studies both in solution and in the solid state. The methods used include various irradiation techniques employing both polychromic and monochromatic light sources. Chromatographic and spectroscopic techniques (UV-visible, infrared, fluorescence and electron spin resonance spectroscopy) were used to study the kinetics of the photochemically induced changes. The effect of sulphonation and bleaching on the yellowing of hig-yield pulps is assessed and the behaviour of active chromophores and intermediates is characterized. Photocycling of chromophoric structures during monochromatic irradiation of mechanical pulps was observed and action spectra of the photoyellowing were constructed. A mechanism for the kinetics of the photo-oxidation of a model system consisting of lignin model compounds in solution is proposed and tested both experimentally and by computer simulation and sensitivity analysis. The behaviour of various lignin model compounds with potential detrimental effects during photoyellowing and applied to pulp matrices was investigated and characterized. Stabilization of mechanical and chemimechanical pulps to light was achieved using polyethylene glycol and ascorbic acid.