Fungi are amongst the simplest of eukaryotes and have become useful paradigms for processes that are fundamental to the way in which higher cells grow, divide, establish for-m and shape and communicate with one another. Leading the way has been the budding yeast Saccharomy cescerevisiae whom ease of manipulation and accessible systems for sexual and molecular genetics have spearheaded basic investigations into fundamental processes as diverse as the analysis of the cell cycle, to investigations of longevity. Although unicellular fungi are greatly outnumbered in nature by the moulds our knowledge of them is much less developed than in this single yeast species. The true hallmark of the filamentous fungus is the hypha – tubular tip-growing cells that are the constituent components of the fungal mycelial colony. This work is dedicated to the analysis of the filamentous life style of fungi, the elaboration of the branching mycelium and the interactions between fungal mycelia. Mycelial fungi also offer major and exciting opportunities for cell and developmental biologists, physiologists, biochemists and developmental biologists. For example, the fungal hypha and the branching mycelium is an excellent system in which to explore the regulation of polarized cell growth, intracellular transport and how nuclei interact within a common cytoplasm, how genetically similar and dissimilar species interact and recognize one another and how growth responses can be coordinated as an organism explores and infiltrates a heterogeneous environment. These themes form the rationale for this work.