Diagnostic testing plays a key role in a surveillance program as diagnostic testing aims to accurately determine the infection or disease status of an individual animal. Diagnostic assays for AIV can be categorized into four broad types: tests for detecting the virus, its antigen, its genomic material, and antibodies to the virus. Virus characterization almost always follows virus detection. The present article surveys the current literature on the goals, principles, test performance, advantages, and disadvantages of these diagnostic assays. Virus isolation can be achieved using embryonating eggs or cell cultures in a lab setting. Virus antigens can be detected by antigen-capturing immunoassays or tissue immunoassays. Viral RNA can be detected by PCR-based assays (gel-based reverse transcription-polymerase chain reaction (RT-PCR), or probe or SYBR? Green-based real-time RT-PCR), loop-mediated isothermal amplification, in situ hybridization, and nucleic acid sequence-based amplification. Antibodies to AIV can be detected by ELISA, agar gel immunodiffusion, hemagglutination inhibition, and microneutralization. Avian influenza virus can be characterized by hemagglutination inhibition, neuraminidase inhibition, sequencing (dideoxynucleotide chain-termination sequencing, next-generation sequencing), genetic sequence-based pathotype prediction, and pathogenicity testing. Novel and variant AIVs can be recognized by DNA microarrays, electron microscopy, mass spectroscopy, and Biological Microelectromechanical Systems. A variety of diagnostic tests are employed in AIV surveillance and monitoring. The choice of their use depends on the goal of testing (fit for purpose), the time of testing during the disease, the assay target, the sample matrix, assay performance, and the advantages and disadvantages of the assay. The article concludes with authors´ perspective of the use of diagnostic assays in the surveillance and monitoring of AIV in poultry.