In the last 20 years, most hematological malignancies have been classified as clinicopathological, each with its own combination of clinical, morphological, immunophenotypic, and molecular genetic features. Molecular and cytogenetic abnormalities can be detected by a wide variety of techniques, from conventional karyotyping to single nucleotide polymorphism analysis. It is necessary to improve our knowledge about cancer in the direction of getting faster results in cancer prevention and treatment. The most effective way to fight cancer is to prevent the disease from occurring. The second most effective way is to detect tumor growth at an early stage before it becomes more malignant. It is very important to understand the molecular mechanism of the disease in this process from diagnosis to treatment. There are different cytogenetic techniques and different therapeutic approaches applied to different types of malignancies. In this section, cytogenetic techniques used for diagnosis and molecular mechanisms and therapeutic approaches in some hematological malignities such as Chronic Myeloid Leukemia, Acute Lymphoblastic Leukemia, Polycythemia Vera and Acute Myeloid leukemia are discussed.