The propagation of causal influences through space-time seems to play a fundamental role in scientific explanation. Taking as a point of departure a basic distinction between causal interactions (which are localized in space-time) and causal processes (which may extend through vast regions of space-time), this paper attempts an analysis of the concept of causal propagation on the basis of the ability of causal processes to transmit “marks.” The analysis rests upon the “at-at” theory of motion which has figured prominently in the resolution of Zeno's arrow paradox. It is argued that this explication does justice to the concept of the ability of causal processes to transmit causal influence without invoking anti-Humean “powers” or “necessary connections.”