Ovaries have special characteristics in terms of embryogenesis, therefore, their histology and composition are complex and many types of tumors occur. For beginners, whenever they see the pathological classification of ovarian tumors, they often have the experience of “understanding when they read, but forgetting when they close the book”, or they feel more and more confused and do not want to read further. However, the histologic classification of ovarian tumors is closely related to their ovarian manifestations, treatment and prognosis, and it is a basic theory that gynecologists have to learn and understand and cannot avoid. Here I will share with you my learning experience on this chapter: First, let’s learn about the histological composition of the ovary. The surface of the ovary has no peritoneum and is covered by a single layer of cuboidal epithelium, called the germinal epithelium, with a layer of dense fibrous tissue on the deeper side of the epithelium, called the white membrane of the ovary, and then the ovarian parenchyma, which is divided into an outer layer of cortex and an inner layer of medulla. The medulla consists of loose connective tissue and abundant blood vessels, nerves, lymphatic vessels, and a few smooth muscle fibers that continue with the ovarian ligaments. All of these tissue components of the ovary can be tumorigenic, thus forming the corresponding classification of tumor. In view of this, it is not difficult to understand the occurrence of tumors such as fibroma, lymphoma, and primary choriocarcinoma on the ovary. Based on the above ovarian tissue components, a hierarchical understanding from the inside out can be divided into four major categories: surface epithelial-mesenchymal-derived tumors, germ cell-derived tumors, sex cord mesenchymal-derived tumors, and metastatic tumors. 1. Ovarian epithelial-mesenchymal-derived tumors have tissue derived from the ovarian surface epithelium and its underlying mesenchyme, which is continuous with the abdominal mesothelium and is a variant mesothelium with multidirectional differentiation potential. If it differentiates to the fallopian tube epithelium, it forms ovarian plasmacytoepithelial tumors; if it differentiates to the cervical glandular epithelium or intestinal epithelium, it forms ovarian mucinous epithelial tumors; if it differentiates to the endometrial epithelium, it forms ovarian endometrioid tumors (such as the common ovarian chocolate cyst); if it differentiates to the migratory epithelium of the bladder, it forms Brenner’s tumor or migratory cell carcinoma… …, most of these subtypes are further classified as benign, junctional, or malignant according to the degree of differentiation of the tumor. Ovarian germ cell-derived tumors, which originate from germ cells in the ovarian cortex, account for about 30% of ovarian primary tumors, and most of them (95%) are mature teratomas. The cellular differentiation of human syngeneic zygotes occurs early and along 3 major directions, namely the totipotent germ cells (spermatogonia or oogenic cells) for reproduction, the somatic cells (which develop into the ectoderm, mesoderm, and endoderm of the embryo), and the extraembryonic components (which form the trophoblast and yolk sac mesenchyme). Primitive germ cells also have the potential for multidirectional differentiation. Pathology is classified into primitive germ cell tumors and teratomas according to the direction and degree of differentiation of the tumor cells. The former is also known as anterior teratoma, which is malignant; the latter is posterior teratoma, which is mainly benign, and the malignant ones are divided into two groups according to the degree of malignancy: low-grade and high-grade. The primitive germ cell tumor group includes: asexual cell tumor, yolk sac tumor, embryonal carcinoma, polyembryonal tumor, and non-pregnant choriocarcinoma; the teratoma group includes: immature type, mature type, and monodermal type (including ovarian goiter, carcinoid tumor, neuroectodermal tumor, sebaceous gland tumor, and melanocytoma, etc.) 3. Tumors of interstitial origin of ovarian sex cords. The tissue is derived from the ovarian mesenchyme (i.e., tissues other than epithelial cells and germ cells in the ovary). It is believed that granulosa cells are derived from the cortical sex cords, while the supporting cells are derived from the medullary sex cords of mesonephric origin (the ovarian crown is homologous to the epididymis, the ovarian network is homologous to the spermatogenic tubules of the testis, and the ovarian portal cells are similar to the mesenchymal cells of the testis (Leydig cells). During normal embryonic development, the gonadal tissue in the primordial gonads will evolve into the supporting cells of the testicular varicocele in males and into ovarian granulosa cells in females; and the specialized mesenchymal tissue in the primordial gonads will evolve into the mesenchymal cells of the male testis and the vesicular cells of the female ovary. Ovarian gonadal mesenchymal tumors are tumors that evolve from the above mentioned gonadal tissues or specialized mesenchymal tissues, which still retain their respective differentiation characteristics. These tumors have an estrogen/androgen secretion function. The primitive gonadal cells in the developing gonads may differentiate into: 1) the female gonadal-mesenchymal cell type, i.e. into granulosa cells and vesicular cells. When forming tumor, they become granulosa cell tumor and vesicular cell tumor or a combination of granulosa and vesicular cell tumor; (2) male gonadal mesenchymal cells differentiate into testicular support cells and testicular mesenchymal cells, when forming tumor, they become support cell tumor, mesenchymal cell tumor or a mixture of both. 4. Metastatic ovarian tumors All tumors that metastasize to the ovary from other organs are called metastatic ovarian tumors. Because of the rich lymphatic and blood flow of the ovary may be a favorable factor for the ovary to grow metastatic tumors easily. Many carcinomas originating in the gastrointestinal tract and breast often metastasize to the ovary first. Ovarian metastases originating from outside the genitalia have a variety of histological patterns. They can be general adenocarcinomas or mucinous adenocarcinomas. However, the most predominant and morphologically distinctive is the indolent cell carcinoma, also known as mucinous cell carcinoma, or Kuchenberg’s tumor. The above is a little bit of my learning experience, which I hope can be useful for young gynecologic clinicians.