Oppositely charged polymers can undergo the process of complex coacervation, 
which refers to a liquid-liquid phase separation driven by electrostatic 
attraction. These materials have demonstrated considerable promise as the basis 
for complex, self-assembled materials. In this review, we provide a broad 
overview of the theoretical tools used to understand the physical properties of 
polymeric coacervates. In particular, we discuss historic theories 
(Voorn-Overbeek, Random Phase Approximation), and then describe recent 
developments in the field (Field Theoretic, Counterion Release, Molecular 
Simulation, and Polymer Reference Interaction Site Model methods). We provide 
context for these methods, and map out the patchwork of theoretical models that 
are used to describe a diverse array of coacervate systems. We use this review 
of the literature to clarify a number of important theoretical challenges 
remaining in our physical understanding of complex coacervation.