Somatic overgrowth and vascular anomalies are often present at birth but may only be clinically recognized later in life. Affected tissues commonly include veins and arteries, skin, adipose tissue, bone, and brain, resulting in a broad phenotypic spectrum encompassing vascular malformations, lipomatous and melanocytic nevus syndromes, skeletal abnormalities, and brain malformations. These conditions are typically caused by postzygotic variants present in a subset of cells due to mosaicism. The developmental timing and cell lineages involved determine the extent and distribution of affected tissues.