We shall be concerned, in this essay, with communicative acts involving at least two persons, whereby a relationship is established between them, based on the use of language in a given context. Such a relationship is often considered to require or contain some measure of 'understanding' between the persons engaged in it, but it also allows for a considerable amount of 'misunderstanding'. Our purpose is to clarify these two notions. In so doing, we will identify and describe two distinct modes of understanding, which we call 'comprehending' and 'grasping'. Our analysis here refers basically to the linguistic interaction between adult speakers belonging to the same linguistic community, in everyday situations. We will occasionally apply our conceptual distinction to other kinds of interaction (e.g. mother/child, husband/wife, patient/analyst, public speaker/audience, etc.), in order to show its ability to provide a means for categorizing different types of dialogues in terms of their differential requirements regarding the two modes of understanding. We shall also suggest a genetical/developmental conjecture to account for the origin and establishment of the two basic modes of understanding.