Repetitive, or multipass, processes are a class of two-dimensional (2D) systems characterized by a recursive action over a finite fixed duration-the pass length. On each sweep, or pass, through the dynamics an output-the pass profile-is produced which acts as a forcing function on, and hence contributes to, the next pass profile. The basic unique control problem is that the output sequence can contain oscillations which increase in amplitude from pass to pass. Differential processes are a sub-class which have well defined structural links with standard linear systems. Previous work has used these links to develop computationally feasible stability tests which can be implemented using only standard linear systems tests. This paper reports the first significant results on the theoretical basis of output feedback based control schemes. The major result is a return difference theory for these schemes.