Dosage compensation serves to equalize X chromosome gene expression in mammalian males and females and involves extensive silencing of the 2nd X chromosome in females. If dosage compensation mechanisms completely suppressed the 2nd X chromosome, then actual physical loss of this “eXtra” chromosome should have few consequences. However, X monosomy has major effects upon normal development, fertility and longevity in humans and some other species. This article reviews observations and arguments attempting to explain the phenotypic effects of X monosomy in humans and other mammals in terms of X chromosome gene dosage.