In humans and other mammals with XX females and XY males, the Y bears a gene (SRY) that induces testis differentiation in the embryo and switches on hormones that masculinize it. This is a very stable sex determining system, but comparison between humans and distantly related mammals reveals that it evolved fairly recently and has undergone major change. Birds, snakes and even monotreme mammals also have stable sex chromosome systems, but the sex chromosomes and sex determining genes quite different. Other retiles, amphians and fish have a great variety of sex determining genes and chromosomes as well as environmentally determined sex, revealing rapid turnover in many taxa. Here I will discuss the evolution of novel sex determining genes and the differentiation of the chromososomes that bear them into differentiated XY or ZW pairs. I will explore what happens to ex-sex chromosomes, and how sex chromosome turnover might impact vertebrate evolution.