The opportunistic fungus Aspergillus fumigatus has emerged as one of the most common fungal human pathogens, causing severe and usually fatal systemic infections that account for more than 200,000 cases annually with mortality rates usually exceeding 50%. During infection, the virulence of A. fumigatus highly depends on its capacity to rapidly respond to external stress encounters in the human niche, such as the host immunological response and the activity of antifungal drugs. The echinocandin, caspofungin, is one of most commonly used antifungal drugs to treat intolerant or refractive patients suffering from invasive aspergillosis. Caspofungin disrupts the catalytic subunit of the Î²-1,3-glucan synthase complex, Fks1, resulting in the reduced production of the main cell wall component of A. fumigatus, the polysaccharide Î²-1,3-glucan. Despite its clinical relevance in patients with aspergillosis, caspofungin displays attenuated activity at high concentrations, a phenomenon known as âthe paradoxical effectâ. Little is known about the paradoxical growth of A. fumigatus during caspofungin treatment. Therefore, in this thesis, I investigated the key cellular and molecular responses of A. fumigatus upon caspofungin treatment, particularly during paradoxical growth by live-cell imaging. High-resolution confocal live-cell microscopy revealed that treatment with either low (0.5 Âµg/ml) or high (4 Âµg/ml) concentrations of caspofungin for 36 h caused similar abnormalities in A. fumigatus, including wider, hyperbranched hyphae, increased septation and repeated hyphal tip lysis. Regenerative intrahyphal growth occurred as a rapid adaptation to the lytic effects of caspofungin on hyphal tips and the dynamic relocation of Fks1 to vacuoles was a key feature observed in response to caspofungin treatment. The reduced amount of Î²-1,3-glucan resulting from caspofungin treatment was compensated by increased Î±-1,3-glucan and chitin content in mature hyphal tips. Interestingly, all lysed cells recovered by regenerative intrahyphal growth. However, after 48 h treatment, only cells exposed to high caspofungin concentrations developed paradoxical growth in leading hyphae. This response was associated with a relocalization of Fks1 at hyphal tips. Consistently, cells undergoing paradoxical growth showed normal morphology and ceased to undergo cell lysis, as well as having a normal content of Î²-1,3-glucan and Î±-1,3-glucan but not chitin, which remained high. Notably, the localization of the regulatory subunit of the Î²-1,3-glucan synthase complex, Rho1, was unaffected by caspofungin, but it was required for the development of paradoxical growth. Interestingly, the gene expression of the Î²-1,3-glucan synthase complex was downregulated by caspofungin treatment. In addition, caspofungin activity induced the nuclear translocation of the Ca+2 regulated transcription factor CrzA to nuclei and only hyphal tip cells in which this translocation occurred underwent cell lysis. Finally, similarly high concentrations of caspofungin also induced paradoxical growth of Aspergillus fumigatus during human A549 alveolar cell invasion. This thesis outlines several critical adaptations that occur at the cellular, subcellular and molecular levels at different times during exposure to high and low concentration of caspofungin.