Toxicology has been defined as the corpus of science devoted to the study of the harmful effects produced by chemical compounds on living organisms. This definition became wider some decades ago with the aim of integrating toxicology and ecology in what is called ecotoxicology or environmental toxicology. There are only slight differences between both “words.” Ecotoxicology intends to link the effects of pollution at different levels of biological complexity, from molecular to population level passing through cell and tissue levels, while environmental toxicology is more focused on the effects produced at the individual level and below. In any case, the tissue interface is placed in the crossroad between all the functional levels mentioned beforehand. Thus, the use of histological approaches to study the normal status and the altered structure of given tissues of aquatic animals affected by disease (histopathology: histo, tissue; pathos, disease; logos, study) has been continuously increasing in the last years and has become a common endpoint in (eco)toxicity bioassays and (active and passive) biomonitoring programs. The histopathological evaluation of the effects produced by toxic substances or pathogens gives information on the target cells and/or tissues of animals affected by those compounds or their derivatives or environmental stressors (not necessarily chemicals), and can be very useful to make a diagnosis and to determine the severity and progress of the disease.