The NS1 protein of influenza A viruses is a non-essential virulence factor with multiple functions during infection. It is primarily known for inhibiting host immune responses, particularly by limiting interferon (IFN) production and the antiviral effects of IFN-induced proteins like PKR and OAS/RNase L. However, NS1 also modulates other aspects of the viral replication cycle, including RNA replication, protein synthesis, and host-cell physiology. This review summarizes the current understanding of NS1's multifunctional roles, including its biochemistry, structure, and intracellular localization. NS1 interacts with various host-cell proteins and RNAs, and its functions may vary between different influenza A virus strains. The NS1 protein is involved in regulating viral replication mechanisms, host innate and adaptive immune responses, and cellular signaling pathways. It also contributes to the pathogenicity of human and animal influenza A viruses. Future studies on NS1 may lead to practical applications such as the design of influenza vaccines, the development of antiviral drugs, and the use of oncolytic influenza A viruses as potential anti-cancer agents. NS1 is synthesized from a collinear mRNA derived from vRNA segment eight and is expressed at high levels in infected cells. It has a strain-specific length and can interact with various cellular proteins. NS1 is divided into two functional domains: an N-terminal RNA-binding domain and a C-terminal effector domain. The RNA-binding domain binds to RNA in a sequence-independent manner, while the effector domain mediates interactions with host-cell proteins. NS1 is primarily localized to the nucleus but can also be found in the cytoplasm. It contains nuclear localization sequences (NLS) and a nuclear export sequence (NES). NS1 plays a role in regulating viral RNA synthesis, mRNA splicing, translation, and the host's innate immune response. It also limits the activity of PKR and OAS, which are key antiviral proteins. NS1 can also inhibit the host's RNAi pathway and contribute to the host's adaptive immune response by limiting the maturation of dendritic cells. NS1 has both pro- and anti-apoptotic functions, and its ability to activate the PI3K signaling pathway may contribute to its role in limiting apoptosis. Overall, NS1 is a multifunctional protein that plays a critical role in the survival and pathogenicity of influenza A viruses.The NS1 protein of influenza A viruses is a non-essential virulence factor with multiple functions during infection. It is primarily known for inhibiting host immune responses, particularly by limiting interferon (IFN) production and the antiviral effects of IFN-induced proteins like PKR and OAS/RNase L. However, NS1 also modulates other aspects of the viral replication cycle, including RNA replication, protein synthesis, and host-cell physiology. This review summarizes the current understanding of NS1's multifunctional roles, including its biochemistry, structure, and intracellular localization. NS1 interacts with various host-cell proteins and RNAs, and its functions may vary between different influenza A virus strains. The NS1 protein is involved in regulating viral replication mechanisms, host innate and adaptive immune responses, and cellular signaling pathways. It also contributes to the pathogenicity of human and animal influenza A viruses. Future studies on NS1 may lead to practical applications such as the design of influenza vaccines, the development of antiviral drugs, and the use of oncolytic influenza A viruses as potential anti-cancer agents. NS1 is synthesized from a collinear mRNA derived from vRNA segment eight and is expressed at high levels in infected cells. It has a strain-specific length and can interact with various cellular proteins. NS1 is divided into two functional domains: an N-terminal RNA-binding domain and a C-terminal effector domain. The RNA-binding domain binds to RNA in a sequence-independent manner, while the effector domain mediates interactions with host-cell proteins. NS1 is primarily localized to the nucleus but can also be found in the cytoplasm. It contains nuclear localization sequences (NLS) and a nuclear export sequence (NES). NS1 plays a role in regulating viral RNA synthesis, mRNA splicing, translation, and the host's innate immune response. It also limits the activity of PKR and OAS, which are key antiviral proteins. NS1 can also inhibit the host's RNAi pathway and contribute to the host's adaptive immune response by limiting the maturation of dendritic cells. NS1 has both pro- and anti-apoptotic functions, and its ability to activate the PI3K signaling pathway may contribute to its role in limiting apoptosis. Overall, NS1 is a multifunctional protein that plays a critical role in the survival and pathogenicity of influenza A viruses.