Although cancer in children is rare, it is the second most common cause of childhood mortality in developed countries. It often presents with nonspecific symptoms similar to those of benign conditions, leading to delays in the diagnosis and initiation of appropriate treatment. Primary care physicians should have a raised index of suspicion and explore the possibility of cancer in children who have worrisome or persisting signs and symptoms. Red flag signs for leukemia or lymphoma include unexplained and protracted pallor, malaise, fever, anorexia, weight loss, lymphadenopathy, hemorrhagic diathesis, and hepatosplenomegaly. New onset or persistent morning headaches associated with vomiting, neurologic symptoms, or back pain should raise concern for tumors of the central nervous system. Palpable masses in the abdomen or soft tissues, and persistent bone pain that awakens the child are red flags for abdominal, soft tissue, and bone tumors. Leukokoria is a red flag for retinoblastoma. Endocrine symptoms such as growth arrest, diabetes insipidus, and precocious or delayed puberty may be signs of endocranial or germ cell tumors. Paraneoplastic manifestations such as opsoclonus-myoclonus syndrome, rheumatic symptoms, or hypertension are rare and may be related to neuroblastoma, leukemia, or Wilms tumor, respectively. Increased suspicion is also warranted for conditions associated with a higher risk of childhood cancer, including immunodeficiency syndromes and previous malignancies, as well as with certain genetic conditions and familial cancer syndromes such as Down syndrome, Li-Fraumeni syndrome, hemihypertrophy, neurofibromatosis, and retinoblastoma.