Polymerized ionic liquids (PILs), a class of polyelectrolytes, are fascinating materials for various applications utilizing ion transport, especially due to their advantageous properties including negligible vapor pressure, low flammability, thermal stability, high conductivity, and wide electrochemical stability windows. Furthermore, PILs are tunable in their properties such as ionic conductivity, glass transition temperature (Tg), solubility, and (electro-)chemical stability. In this thesis, we focus on fundamental studies of phase behaviors of PILs in the solution and bulk states in chapters 2 and 3, respectively. Specifically, non-aqueous systems are used to study the effect of solvent quality in the regime of relatively low solvent dielectric constant, which has not been previously explored experimentally in the field of polyelectrolyte complex coacervation (Chapter 2). From the resulting coacervate phase diagrams in this study, we ascribe the significant differences in salt resistance of coacervates for the two systems as likely reflecting the influence of solvent dielectric constant. The phase behavior of PILs in bulk is also investigated by studying miscibility of oppositely charged PILs with varying charge density and counterion chemistry (Chapter 3). By introducing a miscible ionic pair to a non-charged immiscible polymer blend, miscibility is induced even with a small content (6 – 10 mol%) of charged comonomer. We also show that the mutual miscibility of ionic components manipulates the resulting miscibility of the blends. Finally, we attempt to fabricate a bi-continuous structure of oppositely charged PIL networks using a mesoporous polymer template, which provides an opportunity for developing a new type of electroactive soft actuators with large strain and low operating voltages (Chapter 4).