Trichoderma aggressivum is the causal agent of severe green mould disease in the cultivated mushroom Agaricus bisporus (Samuels et al., 2002). This fungus was First isolated in Ireland in 1987 during an epidemic of green mould disease and has subsequently been isolated from mushroom farms across Europe and North America (Largeteau et al., 2010). There are two subspecies T. aggressivum f. aggressivum and T. aggressivum f. europaeum which are found in North America and Europe respectively, the latter is the subject of this work. The purpose of this study was to investigate the interaction of T. aggressivum with A. bisporus and mushroom substrate, to study the epidemiology of T. ag- gressivum in Bulk Phase III mushroom systems and to develop methods for the detection of T. aggressivum. A separate results chapter is presented for each area of research. T. aggressivum has specific attributes which allow it to grow better in mushroom substrate than other Trichoderma species, resist inhibition by A. bisporus metabolites and ultimately cause more severe reductions in mushroom yield (Largeteau et al., 2000b; Mumpuni et al., 1998). A proteomic method was employed to study the effect of mushroom substrate and A. bisporus tissue on T. aggressivum in vitro. Proteins involved in the oxidative stress response were upregulated in T. aggressivum in both treatments. In vivo the production of oxidative stress response proteins may be part of the ability of T. aggressivum to resist inhibition by the microbial population of mushroom substrate and by A. bisporus, thereby colonising the substrate and reducing mushroom yields (Largeteau et al., 2000b; Savoie et al., 2001a; Williams et al., 2003a). Much of the research on the epidemiology of T. aggressivum in mushroom production was performed in in situ spawn run systems. The Bulk Phase III system has become more common in Europe in recent years but there is little information about T. aggressivum in this system. Cropping experiments were carried out to investigate the epidemiology of T. aggressivum in Bulk Phase III with particular emphasis on the effect of bulk handling of spawn-run substrate on infection. Infection of Bulk Phase III substrate during bulk handling was shwon to be possible. The severity of infection depended on T. aggressivum load and on the amount of mixing performed during bulk handling. Identification of T. aggressivum can be problematic due to morphological similarities with Trichoderma harzianum and other Trichoderma species found on mushroom farms (Seaby, 1996a). The most common method for differentiating T. aggressivum from other species is the of PCR (Chen et al., 1999a) with selectve primers. In this study a real-time PCR method for identification of T. aggressivum was developed, novel selective primers were designed and compared to existing T. aggressivum selective primers and HPLC analysis of secondary metabolite profiles was assessed as an alternative method for the identification of T. aggressivum.