"I think the [21st] century will be the century of complexity" - Stephen Hawking Complexity science is the science of collectives, studying how large numbers of components can combine to produce rich emergent behaviours at multiple scales. Complex systems are not opposed to simple systems, but to separable systems. Their study therefore requires a collective science, often studying a problem across scales and disciplinary domains. The mission of npj Complexity is to provide a home for research on complex systems at the interface of multiple fields. The journal is an online open-access venue dedicated to publishing high quality peer-reviewed research in all aspects of complexity. We aim to foster dialogue across domains and expertises across the globe. At npj Complexity, we publish high-quality research and discussion on any aspect of complex systems, including but not limited to: network science, artificial life, systems biology, data science, systems ecology, and social complexity. Research articles may be based on any approach, including experiments, observational studies, or mathematical and computational models. We particularly encourage studies that integrate multiple approaches or perspectives, and welcome the presentation of new data or methods of wide applicability across domains. It is therefore of critical importance that contributions to npj Complexity be readable to its broad target audience. In addition to publishing primary research articles, we provide a forum for creative discussion of conceptual issues in complexity (see list of Content Types). We welcome Comment articles outlining new important research areas or evaluating the state of related fields and communities, as well as Reviews providing sound syntheses and perspectives on current research.