This dissertation is mainly focused on the effects of Trichoderma spp. on plant performance under abiotic stresses. Chapter one examines the effects of T. harzianum on tomato seed germination and seedling growth under a spectrum of abiotic stresses that commonly happen during seed germination and adversely affect crop performance. Results of these experiments clearly demonstrate that the fungi enhance germination and plant performance under a variety of stresses. It is postulated that Trichoderma may enhance redox buffer capacity of treated plants and thereby enhance their tolerance to a range of abiotic stresses. This hypothesis was examined in chapter two of this dissertation. The expression of genes encoding antioxidants and their activity was shown to be improved by Trichoderma treatment. In parallel, the redox state of major antioxidants, glutathione and ascorbic acid were improved by Trichoderma treatment. The effects of such changes in the antioxidant capacity of plants, on photosynthetic efficiency of tomato plants under stress are examined in chapter three. Finally in chapter four our current understanding of plant-trichoderma interaction is summarized and is compared with that of other known beneficial fungi.