Satellite DNA, once dismissed as junk, is now recognized for important roles despite its rapid evolution and inherent instability. A new review from the lab explores how satellite DNA’s repetitive nature causes replication and repair challenges, leading to copy number instability. Cellular mechanisms, including DNA-binding proteins and repair pathways, help maintain stability. Recent mutation accumulation studies reveal satellite DNA changes over generations, while evolutionary analyses in Drosophila illustrate how selection may shape satellite composition over millions of years.