Polyelectrolyte complexes (PECs) can be formed when mixing together two oppositely charged polyelectrolyte solutions. These ionic assemblies have received numerous interests not only because of the critical role they play in natural systems, but also due to their broad utilization in many industrial areas. Chapter 1 of this dissertation summarized important applications of PECs and highlighted recent advances in understanding the physical behaviors of PECs.The physical states of PECs can range from glassy solids to low viscosity liquids, controlled by the nature of their polyelectrolyte components and the external environmental conditions. For strongly interacting PECs that start as solids under the salt-free condition, salt addition can drive the phase transition from solid into liquid. However, the molecular details of this transition still remain poorly understood. In Chapter 2, we comprehensively explored the solid-liquid transition of a model PEC system formed by two symmetric styrenic polyelectrolytes, from the aspects of dynamics, phase behavior, and internal structures. Rheological measurements revealed an unexpected salt-stiffening trend in the solid states, which was later attributed to dehydration in the complex indicative of osmotic deswelling. From small-angle X-ray scattering and cryogenic TEM images, we also proposed a structural evolution model of this model PEC system. In the solid state, polyelectrolyte chains formed tightly coiled spherical clusters that loosened and expanded upon salt doping. As the PEC transformed into a viscoelastic liquid, polyelectrolyte chains rearranged into ladder-like structures. Even though strongly interacting PECs display exceptional versatility upon salt doping, they have an obvious shortcoming. They usually demonstrate high salt stability to common monovalent salts, causing huge difficulties in accessing the entire solid-liquid-solution spectrum. Chapter 3 introduced a novel approach of decreasing the amount of salt needed to drive this PEC through phase transitions by adjusting solvent quality. We switched the solvent from pure water into binary mixtures of ethylene glycol/water or ethanol/water, enabling the systematic control of solvent hydrophobicity by tuning the ratios between the two solvent components. We discovered that significantly less NaBr was needed to drive this PEC through phase transitions when solvent hydrophobicity increased. Chapter 4 continued to probe the mutual effect of salt and solvent on the behaviors of the model PEC system across the complex-coacervate continuum. We systematically prepared this PEC in two types of salt and four organic cosolvents. Through monitoring the physical states of this PEC with microscopy and cross-comparing the amount of salt needed to drive phase transitions, we developed a general correlation between this salt threshold and the solubility of salt in solvent. Interestingly, at high salt and high organic solvent conditions, an unexpected reverse phase transition occurs from liquid back to solid was captured in selective cosolvent systems, which was traced to the collapse of single polyelectrolyte chains as its origin. These findings provide useful new strategies for controlling the interplay between salinity and solvent quality, and thus enriching potential applications for both bulk macroscale PEC and multilayer complex assemblies.