Although the original description and the terminology of yolk sac tumor (YST) are generally associated with the ovary, histologically identical neoplasms were identified and described in the testis many years earlier. The first case of a testicular tumor histologically corresponding to YST and occurring in an infant was reported briefly by Clark in 1900. Ten years later White (1910) reported in detail another case under the descriptive term “carcinoma myxomatodes of the testis occurring in infancy.” The histological diagnosis of this neoplasm was confirmed by Teoh et al. (1960), who were able to examine sections from the original tumor. In the ensuing years this tumor became known under the terms “adenocarcinoma of the infant’s testis” or “testicular adenocarcinoma with clear cells occurring in infancy” (Magner et al. 1956). The tumor was considered specific and exclusive to infancy and early childhood, but its histogenesis remained unclear. Teratomatous or germ cell origin was considered a possibility (Mostofi 1952), but an origin from rete testis, efferent ducts, mullerian remnants, or mesothelium was also postulated (Magner et al. 1956). In 1960 Teoh et al. in a comprehensive study described 15 cases and reviewed the world literature. After carefully considering the suggested histogenetic theories, they rejected all of them and proposed an origin from embryonic seminiferous tubules. Teoh et al. (1960) regarded this tumor as a malignant embryonic tumor of the testis analogous to renal nephroblastoma and hepatic hepatoblas-toma, and in keeping with this terminology coined the term “orchioblastoma” to describe it. This term became widely accepted, especially in Europe and was in use for more than a decade. The views concerning the histogenesis of this tumor proposed by Teoh et al. (1960) also became at least provisionally accepted in Europe (Collins and Pugh 1964). At the same time a number of investigators in the United States (Abell and Holtz 1963; Houser et al. 1965; Ravich et al. 1966) considered this tumor a neoplasm of teratomatous or germ cell origin and as a specific type of embryonal carcinoma occurring in the testis of infants and young children. Other investigators (Huntington et al. 1963) noted the remarkable similarity between the testicular tumors of this type occurring in infants and young children, and the ovarian and occasional testicular neoplasms occurring in adults described by Teilum (1959) under the term “endodermal sinus tumor” (EST), which later became known as YST. Teilum (1944, 1946, 1950) in a series of papers provided evidence which led to the establishment of EST as a specific histopathological entity and to the elucidation of its germ cell origin. In these studies Teilum was also the first to note the remarkable homology between ovarian and testicular germ cell neoplasms in general and EST (YST) in particular. Following the general acceptance of EST as a malignant germ cell neoplasm of the ovary (Scully 1963; Teilum 1968), and the recognition of similar neoplasms in the testis of young children and adults, the pendulum began to swing toward the acceptance of all corresponding testicular tumors as being examples of a single histopathological entity, irrespective of the age of the patient, and being histogenetically of germ cell origin (Pierce et al. 1970; Wurster et al. 1972; Woodtli and Hedinger 1974; Talerman 1974, 1975; Brown 1976). Further support for these views came from reports describing the occurrence of similar neoplasms in extragonadal locations along the line of migration of the primitive germ cells from the wall of the yolk sac to the primitive gonad. Such tumors were noted in the mediastinum, parapineal region, sacrococcygeal region, and retroperitoneum (Teilmann et al. 1967; Bestie 1968; Huntington and Bullock 1970; Teilum 1971). Studies showing that YST (EST) is associated with synthesis of α-fetoprotein (AFP) (Tsuchida et al. 1973; Talerman and Haije 1974; Norgaard-Pedersen et al. 1975) provided additional evidence of yolk sac origin. AFP production by normal fetal yolk sac had been conclusively demonstrated a few years earlier (Gitlin and Pericelli 1970; Gitlin et al. 1972).