The greatest challenges faced by society lay at the interface between traditional scientific disciplines--chemistry and economics, social sciences and computer science, physics and biology. A common characteristic of these challenges is that they involve many components that influence each other in nontrivial ways; in this sense, these are complex systems. With many recognized researchers in complex systems, Catalonia has become an international hub for complexity science. COMPLEXITAT is the Catalan Network for the Study of Complex Systems. COMPLEXITAT brings together researchers from disparate disciplines with the goal of promoting the research and popularization of complexity science in Catalonia.