According to a worn-out cliché, gel is a material that is easier to recognise than to define. Fortunately, I do not aspire to reach universal definitions and conclusions. The concept of hydrogel that this thesis applies to is a material that consists of a network of permanently cross-linked polymer chains filled with water or an aqueous solution. The polymer may carry charges along its chain, in which case it is called a polyion and will be closely associated with counterions to neutralise the charge, together termed a polyelectrolyte. The gel network can be formed by vulcanisation of a polymer melt or by a polymerisation reaction in the presence of a cross-linking monomer, forming a mass of partly entangled, partly cross-linked polymer strands. If miscible with water, such a network will readily absorb water and swell. In many ways it is equivalent to a solution of linear (i.e. not cross-linked) polymer, but one that cannot dissolve or rearrange on long scale. Instead it acts like a rigid body that remembers its shape, even upon swelling. Even though it cannot dissolve, the solubility of the polymer backbone affects how much water will enter. The equilibrium volume of the material is thus determined by the pressure from water trying to enter it, i.e. the osmotic pressure, and any change in the osmotic pressure causes a volume transition. The transition can be continuous, so that any small change in osmotic pressure results in a small change in volume, but it can also be discrete, so that some infinitesimal change in osmotic pressure may cause a finite change in volume. If the latter, the transition may be seen as a volume phase transition.