We speak and understand the same language, but it’s generally assumed that language production and comprehension are subserved by separate cognitive systems. So they must presumably draw on a third, task-neutral cognitive system (“grammar”). For this reason, comprehension-production differences are a thorn in the side of anybody who might want to collapse grammar and language processing mechanisms (i.e., me!). In this talk I will explore two linguistic domains from the perspective of comprehension and production. In the case of syntactic categories, I will show that the same underlying mechanisms can have rather different surface effects in comprehension and production. In the case of argument role information, I will show an apparent conflict between comprehension and production. In production, argument role information tightly governs the time course of speech planning. But in comprehension, initial prediction mechanisms seem to be blind to argument role information. I argue that both the similarities and contrasts can be captured under a view in which the same cognitive architecture is accessed based on different information, i.e., sounds for comprehension, messages for production.