Fission theory first developed within the framework of the liquid drop model. Shell model concepts were introduced into fission theory much later than they were in nuclear structure theory, but then with spectacular success in explaining striking experimental results then emerging in actinide fission. In the last two decades the complex topography of the fission barrier that is the result of shell model theory has been a major theme in the expanding knowledge of fission, most experimental data finding a natural explanation within this theme. The development of the concept of shell model structure in the fission barrier is outlined in this review.