An update on harmonic maps of finite uniton number, via the zero curvature equation

This is primarily a survey of the developments in the theory of harmonic maps of finite uniton number (or unitons) which have taken place since the introduction of extended solutions by Uhlenbeck. Such maps include all harmonic maps from the two-sphere to a compact Lie group or symmetric space. Extended solutions are equivalent to harmonic maps, but the advantage of extended solutions is the fact that they are solutions to a zero curvature equation. The closely related complex extended solutions (or complex extended frames) are in fact even more advantageous, as has been pointed out by Dorfmeister, Pedit and Wu. We use them to review the existing theory and also to prove some new results concerning harmonic maps into the unitary group.