The ability of light to affect and modify human activity, both physiologically and psychologically has been known for many centuries. The treatment of wounds with light with varying properties has seen a dramatic rise over the last twenty years, from initial reports of the "magic laser" in the 1970's, to recent well controlled clinical trials. However, one of the major problems facing researchers and clinicians in elucidating the mechanisms by which light can modulate wound repair, is that of the different parameters of the light sources used, be they true laser devices, monochromatic light sources or. broadband wavelength units. This thesis describes a series of in vitro studies of the effects of varying the parameters of three light sources a) directly on fibroblast proliferation and b) indirectly, via supernatant from light-irradiated macrophage-like U-937 cells. The effect of U-937 cells was investigated because, as well as playing a pivotal role in wound debridement, macrophages playa central role in mediating the body's inflammatory and immune responses, mainly through the release of various polypeptide growth factors and cytokines and these can modulate wound healing. The experiments reported here show that there is a clear dose-response effect on the proliferation of fibroblasts when grown in macrophage-conditioned medium in which U-937 cells had been subjected to energy densities of 2.4, 4.B and 7.2J/cm2 using a monochromatic light source of 660nm at a frequency of 5kHz. Earlier work had shown that of a number of wavelengths examined, this wavelength evoked the greatest response. 2.4, 4.Band 7.2J/cm2 produced an increase in fibroblast proliferation above that of the sham-irradiated sample, 7.2J/cm2 producing the greatest effect. 9.6 J/cm2 was found to be inhibitory. The relationship between energy density and power density was investigated using the same model, but with a pure laser light source (820nm, frequency 5kHz). The U-937 cells were exposed to either 400mW/cm2 or 800mW/cm2 at energy densities of either 2.4 or 7.2J/cm2 (the doses giving the lowest and highest response in the previous experiment). There was a statistically significant difference in fibroblast proliferation between the 400mw/cm2 and Boomw/cm2 treatments, BoomW/cm2 producing the greatest cell proliferation at an energy density of 2.4J/cm2• There was no significant difference between the sham-irradiated sample and the 400mw/cm2 treated group. In contrast, using an energy density of 7.2J/cm2, the 400mW/cm2 treatment produced a greater increase in fibroblast proliferation than the sham-irradiated sample and there was no significant difference in cell number between the sham irradiated sample and 800mw/cm2 sample. The 400mW/cm2 sample produced a greater increase in cell proliferation than the 800mW/cm2 sample. Using the same model as above, the effects of two broadband (400-2000nm, continuous wave) light sources, of which one was 95% and the other 14% polarised, were investigated using energy densities of 1.44 and 2.88 Jjcm2• The proliferative response was greatest in the cultures exposed to supernatants from macrophages treated with the 95% polarised light source for both the energy densities used. The direct effect of 860 nm laser light (continuous wave) on the proliferation and succinic dehydrogenase levels of human forearm fibrobasts was investigated using energy densities of 2 and 16 Jjcm2• At an energy density of 2Jjcm2, succinic dehydrogenase and fibroblast proliferation levels increased to above that of the sham-irradiated sample, however, at an energy density of 16Jjcm2, succinic dehydrogenase levels and fibroblast proliferation were inhibited. The results presented in this thesis indicate either stimulation or inhibition of cellular activity which can be induced by specific levels and types of light irradiation and helps to elucidate the mechanism of action of light-producing devices at the cellular level.