This review presents an overview on the research on pH-responsive microgel particles in the last 10 years. Microgels are cross-linked latex particles that are swollen in a good solvent. Significant quantitative studies have been conducted to investigate the swelling behavior (microscopic) and rheological (macroscopic) properties of the pH-responsive microgel particles as a function of neutralization degree, ionic strength, and cross-linked density. Mono-dispersed, alkali-swellable microgels containing carboxylic acid lattices, whose properties display extreme pH sensitivity in water is considered in detail in terms of swelling behavior and rheological properties. Their stability in solution and ability to undergo reversible volume phase transitions in response to pH makes them ideal model systems for the development of a semi-empirical as well as theoretical approach for predicting the viscosity of dilute and concentrated hard and soft sphere systems. The review concludes with a discussion of some recent applications of pH-responsive microgel particles.