Disease surveillance is a staple programme in most developed poultry systems and is a cooperative effort between individual poultry companies, regional and national disease control agencies and programmes, public and private diagnostic laboratories, research universities, wildlife agencies and allied animal health industries. Surveillance can be active in the form of sacrificing birds for clinical necropsy examination or submitting samples for diagnosis during a disease outbreak, or passive in the form of routine diagnostic assays designed to provide a historical picture of a flock or region. This chapter reviews both types of surveillance, covering diagnostic laboratories, diagnostic assays, high-quality sampling and submission, and epidemiology.