The use of tacit knowledge is a common feature in everyday communication. It allows people to communicate effectively without forcing them to make everything tediously and painstakingly explicit, provided they all share a common understanding of whatever is not made explicit. If this latter criterion does not hold, confusion and misunderstanding will ensue. Tacit knowledge is also commonplace in requirements where it also affords economy of expression. However, the use of tacit knowledge also suffers from the same risk of misunder-standing, with the associated problems of anticipating where it has the potential for confusion, and of unraveling where it has played an actual role in misunder-standing. Thus the effective communication of requirements knowledge (whether verbally, through a document or some other medium) requires an understanding of what knowledge is and isn?t (necessarily) held in common. This is very hard to get right as people from different professional and cultural backgrounds are typically involved. At its worst, tacit requirements knowledge may lead to software that fails to satisfy the customer?s requirements. In this chapter we review the diverse views of tacit knowledge discussed in the literature from a wide range of disci-plines, reflect on their commonalities and differences, and propose a conceptual framework for requirements engineering that characterizes the different facets of tacit knowledge that distinguish the different views. We then identify methodolog-ical and technical challenges for future research on the role of tacit knowledge in requirements engineering.