The INHAND Project is a joint initiative of the societies of toxicologic pathology from Europe, the United Kingdom, Japan, and North America to standardize diagnostic nomenclature and criteria used in toxicologic studies. The INHAND initiative includes recommended nomenclature for evaluating histologic specimens from nonclinical studies involving laboratory animals including rodents, non-human primates, dogs, minipigs, rabbits, and fish. Specific terminology and criteria are derived from the consensus opinions of senior toxicologic pathologists and subject matter experts who have expertise in the different species of interest. The standardized nomenclature presented in this document is also available electronically on the internet (http://www.goreni.org/). Sources of material included government databases, including the Registry of Tumors in Lower Animals (RTLA), academia, and industrial laboratories throughout the world. This introduction provides context for 14 chapters, arranged by organ system, that define the INHAND nomenclature and diagnostic criteria for fish used in nonclinical studies. Additionally, the current introductory chapter describes both general features of INHAND methodology as well as elements characteristic of toxicologic fish studies. The latter includes fish study design and conduct; euthanasia, sampling, the histologic processing of fish tissues, and a general approach to evaluating fish studies.