Influenza A viruses (FLUAV) have a wide host range. While its natural hosts are considered to be aquatic bird of the genus Anseriformes and Charadriiformes, including ducks, geese, gulls, and swans, among others, FLUAV can infect various animals, including bats, dogs, horses, chickens, and pigs. Pigs are considered a “mixing vessel” host because they can be infected with FLUAV from multiple species, which can promote the emergence of novel zoonotic FLUAVs with pandemic potential as occurred with the 2009 pandemic H1N1 virus. Nonetheless, human-to-swine transmission events are more common than zoonotic infections. Despite many of these human-origin viruses becoming established in swine and contributing to the diversity of viruses circulating in pigs globally, little is known about the biological processes driving human-to-swine adaptation of FLUAV. The studies presented here aimed to evaluate the molecular evolution of human-origin H3N2 viruses to the swine host using an in vivo sustained transmission model. Samples collected during the study were sequenced using next-generation sequencing and viral variants that emerged were characterized in vivo and in vitro. The results presented in this dissertation highlight the critical role of the infection of the lower respiratory tract and alveolar macrophages for onward transmission of human-like viruses during adaptation to swine mainly mediated by changes in the HA protein such as the A138S amino acid change. We found an important role of calcium limiting infection of human viruses in pigs, suggesting a novel role of calcium in the host range of FLUAV. This process was shown to me modulated by the neuraminidase low-affinity calcium-binding pocket which is structurally different in human and swine H3N2 viruses leading to enhanced stability of the NA tetramer of swine viruses under calcium-depleted conditions. Additionally, we found that previous immunity leads to a different viral evolutionary pathway compared to non-immunized animals, resulting in selection of HA variants such a V186G and F193Y. This changes in the HA resulted in a reduction of glycans supporting binding of FLUAV which seems to be compensated by increased affinity for sialic acid. The data presented here provides insights into host barriers limiting human FLUAV adaptation to pigs and the complexities associated with the host immune response that influenza A viruses must overcome to become established in the swine population.