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The impact of filamentous fungi on human welfare has never been greater. Fungi are acknowledged as the most economically devastating plant pathogens (1) and are attaining increasing notoriety for their ability to cause life-threatening infections in humans (57, 71), and fungal products sustain a billion dollar manufacturing industry (70). The tools available to study filamentous fungi are more sophisticated than ever and include the complete annotated genome sequences of multiple filamentous fungi (12), resources being made available through various functional genomics projects, and advanced bioimaging methods, including high-resolution live-cell imaging (20, 32) and electron tomography (19, 50). The increasing impact of filamentous fungi, along with the rediscovery of pseudohyphal growth in yeast (22), has focused attention on the molecular mechanisms underlying hyphal morphogenesis. Attempts to understand hyphal morphogenesis have historically followed two different lines of investigation. Microscopists have defined, with increasing detail, the subcellular organization of the hyphal tip. This led to the description of the Spitzenkö̈rper, an apical cluster of vesicles, cytoskeletal elements, and other proteins, which plays a crucial role in hyphal extension (4). Geneticists have identified gene products required for hyphal morphogenesis by characterizing morphological mutants (51, 52). Initial studies in the laboratories of Beadle, Tatum, and colleagues attempted to link morphogenesis to specific biochemical pathways. More recent screens have identified a multitude of signaling and cytoskeletal functions required for hyphal extension (62, 72). In the past few years, comparative genomics efforts have allowed fungal biologists interested in hyphal morphogenesis to exploit the wealth of knowledge about polarized growth in the yeast Saccharomyces cerevisiae. Many informative homologies between filamentous fungi and yeast have been uncovered. Notably, this includes several components of a multiprotein complex termed the polarisome (28), which regulates microfilament formation at polarized growth sites in yeast (61). Perhaps more importantly, several gene products involved in hyphal morphogenesis have been shown to have no homologue outside of the filamentous fungi. This emphasizes the potential novelty of the mechanisms underlying hyphal morphogenesis. In this review, we summarize past efforts to understand hyphal morphogenesis and pose a series of questions designed to focus future efforts in this area.