The majority of testicular cancers are derived from germ cells and these tumours (TGCT) may occur in males of all ages, and each age-related type has a different pathogenesis. Rare infantile TGCT are likely derived from embryonic stem cells or ectopic yolk sac cells, whereas an equally rare TGCT, spermatocytic seminoma of elderly men, is caused by gain-of-function mutations of genes that control proliferation and survival of post- pubertal spermatogonia. TGCT of adolescents and young adults are the most common type and are derived from intratubular carcinoma in situ testis (CIS), After puberty, CIS progresses to a seminoma, heterogeneous non-seminomas (including teratomas) or combined TGCTs. CIS and TGCTs have features of developmentally arrested primordial germ cells or gonocytes, and retain embryonic stem cell-like pluripotency. The arrest of germ cells differentiation is caused by misregulation of fetal testis development and impaired function of somatic cells. In most cases, the misregulation is a sign of the testicular dysgenesis syndrome (TDS), a multifactorial and complex syndrome that has a broad spectrum of phenotypes ranging from moderate impairment of spermatogenesis, problems with testis descent to a variety of disorders of sex differentiation (DSD), the latter caused mainly by gene mutations. The aetiology of testicular cancer and other increasingly common milder phenotypes is unknown but is caused predominantly by environmental/lifestyle factors acting during development. The effects of these factors are likely modulated by genomic variation and epigenetic factors, thus explaining the individual- and population-level differences in the prevalence of testicular cancer.