In 1871 Darwin famously wrote that, in his opinion, there was no fundamental difference between man and the higher mammals in their mental faculties. In the past decades this claim has driven much empirical research and, by and large, the evidence supports Darwin’s hypothesis. One mental faculty, however, has been particularly difficult to study empirically, with little progress made until recently: the faculty of language. Non-human primates notoriously do not speak, and are unable to acquire speech even with substantial training efforts. Yet speech is only one manifestation of human language, a complex behaviour based on a number of fundamental processes: coding, inference, and assessments of common ground. This is a proposal to investigate the biological origins of these processes in the different modalities of non-human primate communication and the underlying social cognition. Although primates have little control over their articulators, they are able to encode information using discrete and graded signals, sometimes composed into more complex sequences. The first line of investigation concerns the flexibility of primate signal production, both at the unit and sequence level, in the visual and vocal domain. The second line deals with the question of semantic content, that is, what types of meaning receivers can extract from signals and sequences, and how they integrate signal structure and sequential composition with pragmatic context and signaller intention. The third line of enquiry is to study the biological origins of common ground, including audience awareness and cooperative motivation during acts of communication. Primate social cognition and communication are intimately intertwined, and the overall aim of this project is to empirically elucidate this crucial intersection to provide a deeper understanding of the primate origins of the human language faculty.