In this thesis, neoplasms of the testicles and prostate in male dogs and neoplasms of the ovaries and uterus in female dogs are isolated and described. The known incidence, breed predisposition, and age predisposition for each of these neoplasms have been determined. The prevalence is challenging to reliably determine due to the large number of castrated dogs. The text then proceeds to describe the tumor development process, involving mutations of protooncogenes and tumor suppressor genes. These mutations occur under the influence of carcinogens, but they can also occur spontaneously, often being hereditary. The TNM classification system is commonly used to assess tumor malignancy. The text further describes types of neoplasms and their main characteristics, such as malignancy grade and hormonal activity. Clinical symptoms and prognosis of disease outcomes are highlighted. The most significant tumors of the testicles in dogs are seminomas, Sertoli cell tumors, and Leydig cell tumors. These are generally well-contained, benign tumors that rarely metastasize. Cryptorchid testicles have a higher likelihood of tumor development, usually Sertoli cell tumors. Prostate neoplasms are very rare but often malignant with high metastatic potential. The most significant is adenocarcinoma, which can metastasize to bone tissue, liver, spleen, lungs, lymph nodes, heart, and kidneys. In cases of suspected prostate tumors, it is important to use rectal palpation to determine changes in the size and shape of the gland. Ovarian neoplasms mainly originate from epithelial, stromal, or germinal cells. They can occur bilaterally or unilaterally, with adenocarcinomas, granulosa cell tumors, dysgerminomas, and teratocarcinomas being clinically significant. Some of them have the potential to metastasize, and some, like granulosa cell tumors, can be endocrinologically active. Uterine neoplasms in female dogs are extremely rare, with the most common being benign leiomyoma. For most benign neoplasms, the prognosis is very favorable after surgical intervention, while for malignant tumors, the prognosis is much worse and depends on the extent of metastasis. The text concludes by describing the most commonly used diagnostic methods and treatment approaches based on the type and behavior of the tumor.