Polyelectrolyte gels have the capacity to absorb large amounts of multivalent species of opposite charge from aqueous solutions of low ionic strength and release them at elevated ionic strengths. The reversibility offers the possibility to switch between “storage” and “release” modes, useful in applications such as drug delivery. The talk describes systems in which so-called volume phase transitions (VPT) of the gel network take place upon the absorption and release of proteins and self-assembling amphiphiles. I will give a short background of thermodynamic driving forces behind complex formation in oppositely charged mixtures, the role played by cross-links in covalent gels, general aspects of phase coexistence in networks in relation to Gibbs’ phase rule, and how we have theoretically modelled the interaction between components. After that I present results from our experimental investigations of systems undergoing collapse and swelling transitions of gels in contact with solution reservoirs of macroions and surfactants. Here I describe our understanding of the conditions required for VPT, competing mechanisms, and hysteresis effects. After that I address equilibrium aspects of core–shell phase coexistence in gels in equilibrium including recent theoretical and experimental studies providing evidence of thermodynamically stable core–shell phase separated states, and analyses of the conditions under which they exist. Finally, we describe the results from investigations of mechanisms and kinetics of the collapse/swelling transitions induced by the loading/release of proteins, surfactants, and amphiphilic drug molecules.