The term Necropsy comes from the Greek nekros: dead and opsis: vista, and is equivalent to “autopsy” in human medicine. It is based on the systematic external and internal examination of structures, organs and tissues, plus the targeted sampling of fluid and/or tissues for subsequent laboratory analysis.

necropsy is applied in a variety of situations including diagnosis, health monitoring, surveillance, experimental research or the assessment of bio-assays or new procedures. It is also a valuable tool for the study and the comprehensive understanding of “normality”, and as such indispensable to differentiate and recognize pathological changes. Given the many different purposes is therefore essential to clearly define the objectives so the number of fish to examine and the protocol to follow can be decided.
Necropsy for diagnostic purposes is probably the most common; it aims to  establish the cause of death or the nature of the morbidity of a fish or group of fish. In addition to the observation and record of the anatomo-pathological characteristics, the necropsy for diagnostics work will normally require sampling of fluids and /or tissues for subsequent analyses (haematology, histopathology, virology, bacteriology, parasitology, molecular genetics or immunology). A diagnosis is then issued after the critical analysis of the case history, clinical observations, anatomo-pathological findings and the laboratory results.
When the necropsy is performed within regulatory frameworks, along with international guidelines, additional procedures, the type and number of samples may be further governed by regional or national regulations. (e.g. European Union EU, or under “additional guarantees”), and procedures may also require  validation and  accreditation by external independent bodies (e.g. International Organization for Standardization ISO, The United Kingdom Accreditation Service UKAS).