Definition, etiology and classification of leukemia, features of acute and chronic leukemia, factors influencing prognosis of leukemia, importance of Philadelphia chromosome and clinical features are described. Laboratory diagnosis including findings of peripheral blood film, bone marrow examination, biochemical tests and other special tests like immuno histochemistry and different markers are mentioned. Differences between lymphoblast and myeloblast are also tabulated. Definition, types and etiology of leukemoid reaction, differences between myeloid and lymphoid leukemoid reaction and with leukemia are also mentioned in tables. Pancytopenia with its causes, peripheral blood film findings, aplastic anemia, etiology bone marrow findings of it are also incorporated. Normal bone marrow findings, indications of bone marrow aspiration, definition of dry tap and blood tap with cause are also elaborated.