Testicular intraepithelial neoplasia (TIN; so-called carcinoma in situ of the testis) is the uniform precursor of testicular germ-cell tumors. TIN is derived from embryonal gonocytes and is present in the testis of a future testis cancer patient at the time of birth. TIN spreads inside the seminiferous tubules until it progresses to invasive cancer. Diagnosis is best achieved by surgical biopsy of the testis and subsequent im-munohistological staining of placental alkaline phosphatase. This enzyme is present in gonocytes, TIN, and seminoma as well as in several other types of germ-cell tumors but not in normal germ cells. TIN is found in testicular tissue adjacent to testicular germ-cell tumors and is observed in all clinical groups known to be at risk for testicular cancer: cryptorchidism (2-3%), infertility (1%), ambiguous genitalia (25%), in contralateral testis of patients with testis cancer (3-6%). Conversely, TIN is not found in the normal male population.If TIN is left untreated, there is a 50% probability of progressing to frank germ-cell neoplasm within 5 years. Localized radiotherapy to the testis with 18-20 Gy eradicates TIN and germ cells while Leydig cells are preserved. The patient can thus be spared orchiectomy and hormone supplementation. The concept of TIN offers the chance of very early detection of testis cancer and organ-preserving early treatment.