Avian influenza (AI) viruses have been isolated from a wide diversity of free-living avian species representing several orders. Isolations are most frequently reported from aquatic birds in the Orders Anseriformes and Charadriiformes, which are believed to be the reservoirs for all AI viruses. Since their first recognition in the late 1800 s, AI viruses have been an important agent of disease in poultry and, occasionally, of nongallinaceous birds and humans. However, the recent highly pathogenic avian influenza (HPAI) H5N1 virus epidemics have increased the awareness of AI viruses and their potential implications among the scientific community, politicians, and the general public. In response to the spread of HPAI H5N1 viruses to Europe and Africa in 2005-2006, many countries developed surveillance plans to detect AI viruses; a large portion of these sampling efforts was targeted at migratory avian species. This chapter is intended to give general concepts and guidelines for surveillance of the AI virus in wild birds. Separate sections are included for low pathogenicity avian influenza (LPAI) and HPAI H5N1 viruses because the unique biological characteristics of HPAI H5N1 require a modified surveillance plan tailored to these viruses.