People with severe gender identity problems have always existed. Some hope to find a solution to their deep intrapersonal conflicts in a so-called sex transformation. The term 'transsexual' was coined by Cauldwell in 1949 and since then an unknown number of people all over the world, probably more biological men than women, have undergone a hormonal and surgical sex-transformation procedure. In this chapter we discuss the impossibility of real sex change from man to woman and vice versa. We briefly touch on two classical cases of sex transformation with some connection with Denmark: the Lili Elbe case of 1930 and the famous Chris Jorgensen case from the early fifties. We discuss the incidence of transsexuality and follow-up studies of patients, and give preliminary results from a Danish study of 110 people (81 men and 29 women) who during the past 25 years have applied for a sex-change operation. Of these, 56 individuals (42 men and 14 women) have had such operations. These are minimum figures since some transsexuals are known to have had operations in other countries but have not contacted Danish hospitals or health personnel.