The vast host range, genetic variability, and zoonotic potential of avian influenza viruses (AIVs) pose a substantial threat to global animal and human health. AIVs are classified based on their hemagglutinin (HA) and neuraminidase (NA) surface proteins into multiple subtypes and further divided into highly pathogenic (HPAIV) and low pathogenic (LPAIV) forms. While LPAIVs often circulate silently in wild waterfowl, HPAIVs, especially H5 and H7 subtypes, can cause devastating outbreaks in poultry with high mortality and severe economic losses. Additionally, these viruses are capable of crossing species barriers, sometimes infecting humans and other mammals, raising the possibility of a pandemic outbreak. Prevention strategies focus on surveillance, biosecurity, and vaccination. Traditional inactivated vaccines, recombinant vector vaccines, and emerging platforms like mRNA-based vaccines are explored as tools for control and pandemic preparedness. Despite control efforts, eradication remains unlikely due to persistent reservoirs in wild birds and the complex ecology of AIVs. AIVs are reviewed in this chapter for their historical emergence, molecular biology, transmission dynamics, and interactions with hosts. Key subtypes like H5Nx, H7Nx, and H9N2 are discussed in terms of epidemiology, evolutionary trends, and impact on both poultry and public health. The chapter also highlights the role of viral proteins in pathogenicity, clinical manifestations, diagnostic approaches, and host immune responses. Additionally, the rapid mutation and reassortment that facilitate the emergence of novel genotypes with enhanced virulence and expanded host range are discussed. The document also underscores the necessity of a One Health approach integrating veterinary, human, and environmental health sectors, to effectively monitor, prevent, and respond to AI outbreaks. Continued research into viral pathogenesis, immune responses, and next-generation vaccine development is crucial to mitigate the risks posed by this ever-evolving virus.