In its classic appearance, Wilms tumor is triphasic and consists of variable proportions of blastema, stroma, and epithelial cells. One component may dominate over others in a given case, resulting in biphasic and monophasic variants. The blastemal component consists of highly cellular sheets of primitive small blue cells with scant cytoplasm. The epithelial component consists of abortive tubules and glomerular structures. Stroma is usually fibroblastic or myxoid and may contain heterologous elements such as skeletal muscle, smooth muscle, bone, cartilage, adipose tissue, and neuroglial tissue.