  These lecture notes describe a basic theory of chemically active droplets,
which are droplets kept away from equilibrium by driven chemical reactions. The
notes assume a basic familiarity with equilibrium thermodynamics of phase
separation, and thus focus on three separate themes, which were discussed in
three separate lectures: (i) The kinetics of phase separation, including the
early-stage dynamics of spinodal decomposition and the late-stage dynamics of
Ostwald ripening. (ii) Transition state theory as a simple,
thermodynamically-consistent kinetic theory of chemical reactions, which
permits explicit driving in open systems. (iii) The combination of phase
separation and reactions, leading to active droplets. We discuss the two
fundamental classes of internally-maintained and externally-maintained
droplets. A simple version of externally-maintained droplets permits an
effective electrostatic analogy, which indicates how the reaction-diffusion
system mediates long-ranged interactions. All these aspects are discussed in
the context of biomolecular condensates.
