Aspergillus fumigatus is the most common and life-threatening aerial fungal pathogen, especially among immunocompromised hosts. Understanding the pathobiology of this pathogen requires investigations both to characterise the immunosuppressive status of the phagocytic cells that usually kill the fungus in the immunocompetent host, and to identify fungal virulence factors that either promote the resistance of conidia to innate immunity or stimulate mycelial growth in the lung parenchyma. Recent findings are presented in this review and show how little we know of the interactions between host and pathogen. Now the genome of A. fumigatus has been sequenced, transcriptome/ proteome-based studies should disclose the multiple facets of A. fumigatus pathogenicity.