  We are learning much about how structure forms, in particular how clusters as
nodes in the cosmic web evolve and accrete matter, and about the physical
processes within these objects. In the next decade, the study of clusters will
enable us to tackle important questions regarding the nature of Dark Matter and
Dark Energy, how clusters co-evolve with super-massive black holes at their
centers, and to advance our knowledge about fundamental plasma astrophysics.
This science white paper outlines the key questions and research opportunities
in cluster astrophysics that are emerging in the coming decade and beyond, and
serves as an overview to other cluster related white papers.
