This chapter discusses oncology. Cancer is the second commonest killer in children less than 15 years of age, second only to accidents. As awareness increases and better diagnostic tools become available, more and more children are diagnosed with cancer. With improved therapeutic modalities, the cure rates for childhood malignancies have increased severalfold in the past few decades. Leukemia is the most common malignancy in children, with acute lymphoblastic leukemia (ALL) comprising about 80% and acute myeloid leukemia (AML) comprising 15%. The remaining 5% are made up of chronic myeloid leukemia and the other rare leukemias. The common differential diagnosis includes viral infections, malaria, atypical immune-mediated thrombocytopenia, drug-induced cytopenia and collagen vascular disorders like systemic lupus erythematosus or rheumatoid arthritis and Langerhans cell histiocytosis.