What is ‘Active Matter’?

Over the past decades, active matter systems have attracted the attention of scientists from different areas, including physics, engineering, biology, and the human sciences. These systems involve agents that convert some type of energy into directed motion. Examples range from swimming microorganisms to crawling cells to synthetic active colloids. A striking feature of active materials is that they are strongly driven out of equilibrium and therefore display a variety of unusual phenomena at the single as well as collective level, which differ drastically from their passive counterparts. In this preface, we provide an overview of the main avenues of research in active matter and bring together the topics addressed in our book. We conclude by discussing future research directions in this rapidly-evolving, interdisciplinary research field.