Polyelectrolyte complexation is a powerful approach to generate dynamic materials with adaptive properties and precision polymer and hybrid protein-polymer nanoparticles, which can be tailored to meet requirements specific to their application in e.g. nanomedicine. In this lecture I will discuss the preparation, properties, and (dis)assembly pathways of hydrocolloids comprising at least two oppositely charged (bio)macromolecules. Particularly appealing of such particles is their modulatory: morphology, size, stability, and function are tunable. Systematic studies of structure-property relations allow tailoring of some of these features as desired. Rational design remains however one of the grand challenges, especially for those systems and conditions where kinetic traps are prominent. I will highlight recent work on complex coacervate core micelles, single-enzyme nanoparticles, and polyelectrolyte complexation out-of-equilibrium, that is, orchestrated by clock reactions and induced by polymerization. The impact of polymer architecture on C3Ms as well as novel routes to generate complex coacervate-based particles with high stability and a tunable lifetime will be discussed. Finally, the opportunities of polymerization-induced electrostatic self-assembly (PIESA) to prepare customized hydrocolloids will be addressed. This technology has recently been introduced as an attractive means to prepare polyelectrolyte complex particles on large scale under mild conditions. Extraordinary morphologies have been reported, but these are often unstable. We recently discovered a novel strategy to regulate the polymerization. I will address how this switchable, electrostatically templated polymerization offers excellent control over the assembly pathway to custom-tailor the hydrocolloids that form and gain more in-depth insight in the PIESA process.