This chapter focuses on the development of customary international law and unpacks the requirement of publicity for state practice. It introduces the different levels of publicity and covertness, and closely examines the role of acknowledgement, justifications, and public knowledge within the requirement of publicity in the light of various approaches to the development of (customary) international law. The chapter illustrates how the requirement of publicity can be unpacked into two main parts, where the first relates to how a state communicates its understanding of its practice in relation to international law, and the second relates to how the act itself and — if available — the justifications provided for it, are known and reacted to by other states and international actors.