The study of the beta globin gene has provided great insights into the mechanisms of gene regulation and expression. In this review, we consider the normal regulation and expression of the beta globin gene and illustrate how the various steps may be affected, providing a basis for understanding the molecular pathophysiology of beta thalassemia. Mutations causing beta thalassemia can be classified as beta0 or B+ according to whether they abolish or reduce the production of beta globin chains. The vast majority of beta thalassemia is caused by point mutations, mostly single base substitutions, within the gene or its immediate flanking sequences. Rarely, beta thalassemia is caused by major deletions of the beta globin cluster. All these mutations behave as alleles of the beta locus but in several families the beta thalassemia phenotype segregates independently of the beta globin complex, and are likely to be caused by mutations in trans-acting regulatory factors.