  This white paper, submitted for the Plasma 2020 Decadal Survey, concerns the
physics of weakly collisional, high-beta plasmas -- plasmas in which the
thermal pressure dominates over the magnetic pressure and in which the
inter-particle collision time is comparable to the characteristic timescales of
bulk motions. This state of matter, although widespread in the Universe,
remains poorly understood: we lack a predictive theory for how it responds to
perturbations, how it transports momentum and energy, and how it generates and
amplifies magnetic fields. Such topics are foundational to the scientific study
of plasmas, and are of intrinsic interest to those who regard plasma physics as
a fundamental physics discipline. But these topics are also of extrinsic
interest: addressing them directly informs upon our understanding of a wide
variety of space and astrophysical systems, including accretion flows around
supermassive black holes, the intracluster medium (ICM) between galaxies in
clusters, and regions of the near-Earth solar wind. Specific recommendations to
advance this field of study are discussed.
