In his now-classic mediation on the sociology of secrecy, Georg Simmel cautioned that while ‘human interaction is conditioned by the capacity to speak, it is [also] shaped by the capacity to be silent’.1 As historians, we are trained to see what is present, what is material, and what has effect. Investigating absence, on the other hand, as rewarding as it can be when we are able to reconstruct the seemingly unknowable, can lead us astray with speculative banalities or even counter-factual histories. Yet, as one manifestation of absence in society – in this case, the absence of knowledge – secrecy has had a fundamental place in the constitution, shaping, and functioning of the premodern and modern worlds. It has operated in many registers and appeared in many forms, such as censorship, coded language, classification regimes, and in oaths promising secrecy. All these modes in which we find practices related to secrecy operated within bureaucracies where the regulation of knowledge was either explicitly or implicitly part of their functioning. In looking at manifestations of absences – in particular, practices designed to regulate and then render knowledge absent – bureaucracies represent an emblematic and instructive site to explore questions on the co-constitution of power and knowledge.2