Questioning is a social action, which is closely related to but nonetheless different from interrogatives in the traditional grammar. Then what is questioning in the sense of a social action? What is the relationship between questioning and interrogatives? In addition to seeking for information, are there any other communicative functions that questioning can fulfill? This paper, using Conversation Analysis as its research methodology, studies questioning from the perspective of epistemics proposed by John Heritage. A definition of questioning is given in accordance with the essence of epistemics, then a thorough discussion of the effect of the conversational economy of information and epistemic gradient on the questioner’s questioning design is conducted. Finally, the assistant communicative functions of questioning are touched upon. We think that probing questioning from the epistemic perspective enables us to investigate questioning as a social action from formal, functional and interactional perspectives.