This thesis deals with intracellular signal transduction. This cellular process is a vital component for communication of cells with their environment within an organism. By signal transmission, stimulis on surface of cells are transmitted to the nucleus and result in a cellular effect, such as cell differentiation or proliferation. A variety of signal components, mostly enzymes, enable these intracellular processes. It is believed that tumor cells such as leukemic cells, show changes in transduction pattern. Therefore, this thesis deals with the etiology, pathogenesis, clinical symptoms, classification, diagnosis, as well as the treatment and prognosis of acute leukemias, as they constitute the most common form of leukemia in children and adolescents. In addition, the method of flow cytometry is described, which is an essential analysis for the diagnosis of leukemias and maybe also considered as analysis for signal pathways-parameters. At last, the principle of signal transduction will be described as well as the three principle pathways, which are of significance in leukemic cells, named JAK-STAT, Raf-Ras-MEK-ERK and PI3K-Akt.