Microgels are porous, elastic networks of cross-linked polymers spanning the nanometer to micrometer size scale. When immersed in a good solvent, they can reversibly swell in response to changes in environmental conditions, such as changes in temperature, solvent quality, pH and concentration. These soft colloidal particles have gained considerable importance since they confer tunable properties to the suspension, making them highly valuable in several technological applications, such as in drug delivery, diagnostic, and cell culture/tissue reconstruction. The exceptional characteristics of functional microgels primarily arise from their dynamic, permeable, network-like architecture, their flexibility and, most importantly, their capacity to swell in different solvents. When swelling or deswelling, microgels either absorb or release solvent, respectively, due to changes in their internal osmotic pressure and interactions, resulting in a size variation of several times their original size. In drug delivery, endogenous stimuli-responsiveness is used to induce microgel swelling and release the cargo as a consequence of the morphological response of the supramolecular assembly. Besides, water-swollen networks present various advantages regarding biocompatibility and solubility due to their high hydrophilicity and low interfacial energy, as well as storage and transport of guest molecules thanks to their large internal voids. Swelling also plays a key role in detection devices since most of the microgel-based sensors rely on their swelling state as sensing mechanism. The capability of functionalizing microgel with swelling responsiveness to determined species allows to quantify the concentration of this species, e.g., by means of determination of the volume variation through the reflectance spectrum or by analyzing the effect of swelling on the microgel net charge. In a polar solvent, microgels that incorporate ionic groups through copolymerization can become charged by releasing counterions into the solution. Charge-induced drug binding of drugs to functionalized microgels is a common mechanism for controlled uptake and release in microgel drug delivery. To guarantee an efficient loading, the functional microgel should not only possess charged groups but also it should exhibit a net charge that fosters the loading when in contact with the drug buffer. Anionic microgels have proven their potential utilization in the uptake and delivery of, e.g., doxorubicin, ferritin among other proteins, while cationic microgels for DNA, RNA and oligonucleotides, which results advantageous for gene delivery systems. A deep understanding of the fundamental mechanisms underlying the swelling of ionic microgels and their interaction with other charged species is crucial for further advance in the field. It has been shown that the interplay of electric repulsion among polyelectrolyte chain segments and the osmotic pressure exerted by the counterions can cause a significant expansion in the microgels when contrasted with electrically neutral counterparts. The presence of salt ions, encompassing those from natural self-dissociation or externally added, increases the population of free co-and counterions, affecting the effective screening of the inherent Coulomb interactions between backbone sites and, consequently, modifying microgel swelling. Furthermore, microgels exhibit a nonzero effective charge that determines the effective microgel-microgel interaction and, hence, the suspension stability, and leads the system to show very interesting phase behavior as compared to usual charged colloidal systems. The fast-expanding synthesis field and the ever-expanding array of potential applications require the application of various polymer theories that facilitate the measurement interpretation, helping not only in a fundamental understanding of microgel phenomenology but also supporting their rational design. The addition of new internal degrees of freedom rela-tive to the polymer network makes the exploration of microgel design an arduous and expensive task to be performed experimentally. The enlargement of the parameter space opens up a wide variety of parametric combinations, resulting in peculiar microgel swelling behaviors that might be related to demanded suspension properties. A predictive accurate swelling model results then of remarkable importance, when planning on-demand microgel design.