The undertow is one of the most important mechanisms for sediment transport in
nearshore regions. As such, its formulation has been an active subject of research for
at least the past 40 years. Still, much debate persists on the exact nature of the forcing
and theoretical expression of this current. Here, assuming linear wave theory and
keeping most terms in the wave momentum equations, a solution to the undertow in
the surf zone is derived, and it is shown that it is unique. It is also shown that, unless
they are erroneous, most solutions presented in the literature are identical, albeit
simplified versions of the solution presented herein. Finally, it is demonstrated that
errors in past derivations of the undertow profile stem from inconsistencies between
(1) the treatment of advective terms in the momentum equations and the wave action
equation, (2) the expression of the mean current equation and the surface shear stress,
and (3) the omission of bottom shear stress in the momentum equation
