We propose a workshop on methods and theories for dealing with complex dynamical systems, and their application in HCI. Such methods are increasingly relevant across a wide range of disciplines which focus on human behaviour, applied to understand the role of context and interactions in the behaviour of individuals and groups, and how they unfold over time. Traditional approaches to quantifying and modelling behaviour in HCI have tended to focus primarily on individuals and components. Complexity methods shift the focus onto interactions between components, and the emergence of behaviour from complex networks of interactions, as for example in Enactivist approaches to cognitive science. While we believe that complexity methods can be highly informative to HCI researchers, uptake in the community remains low due to widespread unfamiliarity.This one-day workshop will introduce, support, and encourage the development and adoption of complexity methods within HCI.Reflecting the multidisciplinary mix within complexity science, we will draw on examples of complexity-oriented theories and methods from a range of disciplines, including Control-Theory, Social Science, and Cognitive Science.Attendees will engage in group discussions and a Q\&A with a panel, and a discussion group will be set up ahead of time to encourage exploratory conversations. In this way, diverse backgrounds can be brought together, matched, and inform one another.