The carp fish are native to Asia and Eastern Europe. Common carp fish (Cyprinus carpio) are wide spread in freshwater of tropic lakes, large rivers in Asia and Europe. Carp fish is most important cultured fish and can be considered as domesticated fish, and these type of fish characterized by high tolerance to PH and oxygen concentration variations of water.Thyroid gland or thyroid tissue is a main part of endocrine system and play an important role in regulation of metabolism, growth and metamorphosis and the tissue is well organized in high vertebrates but in low vertebrates as fish, the thyroid tissue is dispersed or scattered in among several tissues. There were many studies about the function and physiology of thyroid tissues in different types of fishes but there are few studies about the histology and histochemistry of thyroid tissue in carp fish, therefor our work will include the study the distribution, histology and morphometry of thyroid tissue of carp fish in two seasons (winter and summer). In this book we focus the light on the distribution, histology and morphometry of thyroid tissue of carp fish in two seasons as well as the effect of environmental variations.