The communicative effect of a collective message from the Dutch former minister of finance Wouter Bos to inform all his contacts about his new email address is completely different from that of a set of individual messages to the same list. The talk will explain how differences of this kind can be modelled in epistemic logic (the logic of knowledge). A central notion here is common knowledge. We will explain the general framework for describing update effects of messages as mappings on epistemic models (“knowledge models”), and we will give a sketch of some recent work in this area.