The objective of this chapter is to introduce the pediatric oncology. This chapter is divided into four parts acute leukemia, lymphoma, abdominal tumors, and brain tumors. Leukemia can present with myriad of symptoms from pallor to life-threatening tumor lysis syndrome. A high index of suspicion in patients presenting with the following features can help to diagnose leukemia early. The diagnosis of leukemia needs a bone marrow aspiration with special studies like immunophenotyping (done by flow cytometry), cytogenetics in addition to bone marrow morphology to characterize the leukemia. Lymphoma is called the great mimic as its symptoms can be confused with a host of other illnesses. Clinically enlarged lymph nodes are the most common presentation of lymphoma in children. Brain tumors are the most common solid tumor in children. An appropriate and timely diagnosis of a brain tumor is essential to avoid undue morbidity and mortality. Substantial progress in molecular and precision oncology has generated potential for advancement of treatment of tumors previously considered to have very poor outcomes.