Wilms tumor (WT) is the most common neoplasm of the kidney in children. It is an embryologic tumor that histologically mimics renal embryogenesis and is composed of a variable mixture of stromal, blastemal, and epithelial elements. Nephrogenic rests, generally considered to be precursor lesions of the WT, are foci of the embryonic metanephric tissue that persist after the completion of renal embryogenesis. These are classified as perilobar and intralobar based on their location and maybe present as single or multiple foci. Intralobar and perilobar rests and the tumors arising from these rests differ morphologically and are characterized by 2 different sets of genetic abnormalities involving 2 adjacent foci, WT1 and WT2, on the short arm of chromosome 11. WTs arising in the intralobar rests tend to be stromal predominant and have a mutation or deletion of WT1. Germline mutation in WT1 may be associated with syndromic conditions such as WAGR and Denys-Drash syndromes. Perilobar rests and their corresponding tumors usually have loss of imprinting/loss of heterozygosity involving WT2, which contains several parentally imprinted genes. Loss of function of these genes, if present constitutionally, may be associated with Beckwith-Wiedemann syndrome or may result in isolated hypertrophy. Abnormalities in several other genes may also be seen in WT. These include WTX, (on chromosome X), CTNNB1 (chromosome 3), and TP53 (chromosome 17) among others. WT with loss of heterozygosity at 1p and 16q may have poor prognosis, requiring aggressive therapy. Treatment modalities for WT have evolved over many decades, primarily through the efforts of Dr J Bruce Beckwith at National WT study. This work is now being carried out by Children Oncology Group in North America and International Society of Pediatric Oncology in Europe. Although their therapeutic approaches are somewhat different, both have reported excellent results with equally high cure rates.