Complex communication is central to human society, and the evolution of communication complexity has attracted a great deal of attention over several decades. Yet quantifying the complexity of facial communication is generally accomplished through subjective categorisation of expressions into repertoires of discrete expressions, an oversimplification that ignores both intra- and inter-specific variability in expression morphology. In this thesis I aim to link form and function of expressions in crested macaques (Macaca nigra), using an objective tool, the Facial Action Coding System, to measure facial movements based on the underlying muscle contractions, thus obtaining morphological data which can be compared between short-term behavioural contexts, and according to long-term social relationships. First, I demonstrate that the Facial Action Coding System for macaques can be used to code crested macaque expressions, and document minor adjustments to the coding guidelines. Second, I examine the morphological differences of expressions associated with different behavioural outcomes, finding that bared-teeth expressions that precede affiliation, copulation, play, and submission outcomes differ in rates of specific muscle movements, as well as general characteristics of intensity and variability. However, when examining “threat” expressions I find no predictive value of morphological differences. Finally, I investigate general trends of the use of intensity and variability of expressions, finding that these characteristics of expressions vary according to the type of interaction in which expressions are used, and that intensity varies according to the pre-existing dominance relationship of the two individuals involved, but not to the strength of their social bond. Overall, this thesis represents a rare detailed objective analysis of facial expressions in a wild nonhuman primate. My results demonstrate the value of a bottom-up approach and caution against the lumping of superficially-similar expressions into repertoires of discrete variants, illustrate the importance of considering variability and intensity of expressions, and provide evidence for the influence of social factors on the style of communication.