Complex systems of any kind are characterised by autonomous components interacting with each other in a non-trivial way. In this short talk, we discuss how the views on complexity are evolving in fields like physics, social sciences, and computer science, and – most significantly – how they are converging. In particular, we focus on the role of interaction as the foremost dimension for modelling complexity, and discuss first how coordination via mediated interaction could determine the general dynamics of complex software system, then how this applies to complex socio-technical systems like social networks.