Evaluation of damages by parasites in teleosts, even if underextimated in the past, is today one of the most used methods to understand the effective impact of a parasite on a certain ichtyc species. The damage caused by a parasite on the host could be classified into direct, with tissue changes, or indirect, with a decrease of the productive performances. Based on this, a preliminary difference could be traced distinguishing parasites that show a coevolution with their host species, characterized by a low damage, and those parasites that can occasionally infect new host species, showing a high degree of damage. In consideration of the damage, parasites can have different actions on the host: subtractive, irritative, mechanical, traumatic, toxic, dismetabolic, antigenic and foretic. Those parasites able to cause tissue changes must be furtherly classified considering the type of inflammation they evoke on tissues: ulcerous, catarral, haemorrhagic, necrotic and granulomatous. Some parasites are encysted in tissues without any host reaction. The fish response against parasite can be inflammatory, the most frequent, but also hyperplastic, metaplastic, neoplastic and immunitary. The paper goes through different tissue changes due to the main parasites of Mediterranean teleosts.