Seminomas account for about 30-40% of all testicular tumors. These are usually is found in men in their 30s and 40s. The condition is usually localized to the testes, although in about 25% of cases it has spread to lymph nodes. Non-seminomas account for 60% of all testicular tumors; subcategories of these tumors are listed below. Non-seminoma tumors often contain more than one of the following cell types: Testicular cancer is an abnormal, rapid, and invasive growth of cancerous (malignant) cells in the testicles. Embryonal carcinoma (about 20% of testicular cancers) occurs in 20-30 year olds and is highly malignant. It grows rapidly and spreads to the lung and liver. Yolk sac tumor (about 60% of all testicular cancers in young boys). Teratomata (about 7% of testicular cancers in adult men and 40% in young boys). Choriocarcinoma is rare. Stromal cell tumors are a kind of tumor that is made of Leydig cells (testosterone-secreting cells), Sertoli cells (cells where sperm matures), and granulose cells. These tumors account for only 3-4% of all testicular tumors. However, they do make up nearly 20% of all childhood testicular tumors. These tumors may secrete a hormone estradiol that can cause one of the symptoms of testicular cancer, gynecomastia (excessive development of breast tissue). This new book presents leading-edge reseach in the field.