Mixing of oppositely charged macromolecules can result in the formation of polyelectrolyte complexes. This associative process is often accompanied by a phase separation, in which the system demixes into two phases: the polymer-rich polyelectrolyte complex (PEC) phase, co-existing with a dilute supernatant phase which is almost free of the polymers. When the PEC phase is liquid, it is termed a complex coacervate. Complex coacervates have found use in hygiene products, food industry, and they are prospective candidates for medical applications, water purification or development of programmable materials. Furthermore, the mechanism of coacervation inspired a plethora of derived soft materials based on the assembly of polyelectrolytes with other charged objects, such as multivalent macroions, colloids, co-polymers yielding coacervate core micelles or proteins. Naturally, the use of polyelectrolyte complexes in such delicate arrangements requires a high level of fundamental understanding of the underlying physics and chemistry which govern the properties of these materials.