There is often an increasing gap between a programmer′s mental conception of a problem and the realization of a parallel program which solves that problem. To bridge this gap, information about program execution must be displayed. Since a parallel program may be executed on several processors simultaneously, activities may be distributed across both space and time, and therefore simple sequential displays of a program′s execution are inadequate. Intuitively, pictures are much more meaningful than text, particularly with parallel programs where large quantities of information may be generated. In this paper, we describe how the behavior of parallel programs may be graphically interpreted, and discuss current approaches and difficulties in parallel program visualization. A graphical interface designed for languages based on message-passing is also presented.