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Thymus is a central lymphoid organ with an important endocrine function located in the superior mediastinum, but in newborns it caudally reaches to anterior mediastinum. Thymic morphology varies greatly even in the same age group. For instance, in young adults, it is typically bilobed and V-shaped, with two small processes extending into the neck. Anomalies of the thymic shape, size and localization are explained by the disruption of thymic primordia formation and/or disruption of its descent from the neck to the mediastinum. The most common thymic anomalies include anomalies in number of the thymic lobes, ectopic thymus, accessory thymic tissue, thymic aplasia, thymoparathyroid aplasia (DiGeorge syndrome), thymic hypoplasia and hyperplasia. In a case of ectopic thymus, the whole thymus is (most often) located in the midline neck, and no thymic tissue is located in the normal position in the mediastinum. The presence of accessory thymic tissue reflects a failed migration of the whole thymic primordia from the third pharyngeal pouches. It may be found at any level of the pathway of normal thymic descent, from the angle of the mandible to the superior mediastinum. The prevalence of foci of accessory thymic tissue varies according to several authors from 1.8% up to more than 50%. A great controversy about the terms being used in the classification of thymic anomalies arises in current literature. Most of the recent articles use the term "ectopic thymus" in a confusing way, because they call cervical extension of the thymus, accessory cervical lobes of the thymus or accessory thymic tissue also "ectopic thymus". The "genuine" ectopic thymus is a distinct entity from accessory thymic tissue localized in the neck. The term "ectopic thymus" should be reserved for states where only ectopic thymic tissue is present, while no thymus tissue is localized in the normal, mediastinal position. These two entities are differentiated also according to the recommendation of the Terminologia Embryologica (ectopia thymi versus textus thymicus accessorius). A cystic version of accessory thymic tissue localized in the neck is called "congenital cervical thymic cyst". The cystic version of aberrant thymic tissue is more often described in the literature than the "classical" solid form. Thymopharyngeal duct cyst is another special and extremely rare variant of cystic accessory thymic tissue. Only circa 5 cases are described in the literature. Relatively common and asymptomatic anatomical variation is the fusion of thymus with one or more parathyroid glands - much common with the inferior due to common embryonic origin of both organs. [ABSTRACT FROM AUTHOR]