The HER2 gene is responsible for the formation of the HER2 receptor on the surface of epithelial cells. Increased numbers of this receptor are associated with a worse prognosis in cancer. Increased numbers of copies of the HER2 gene occur in about 20-30% of breast cancer patients, so determining HER2 receptor levels is important in the current diagnosis and treatment of breast cancer. One diagnostic technique is the immunohistochemical (IHC) method, which permits indirect measurement of overexpression of HER2 receptors, based on subjective determination of the intensity of the color reaction. Another technique is the use of fluorescent in situ hybridization (FISH), which permits the exact number of copies of the HER2 gene to be specified. Based on the results of FISH tests, patients can be qualified for treatment with antibodies that partially block HER2 receptors. This treatment causes inhibition of tumor growth signals. Determining the HER2 status in breast cancer with the FISH method allows the further progress of the disease to be predicted, the right treatment to be chosen and the response to the treatment to be foreseen. Because of the widespread use of the FISH and IHC methods, comparing the advantages and disadvantages of these two methods seems to be relevant.