Polymer microgels consist of swollen networks of crosslinked macromolecules with particulate dimensions. If these networks exhibit a delicate interplay with their environment that allows them to be swollen and deswollen or to be crosslinked and decrosslinked upon external stimulation, they can serve for a variety of applications in sensing and actuation. Such environmental sensitivity can be realized either by the use of covalently crosslinked polymer networks that exhibit critical miscibility with their swelling medium or by the use of transient and reversible, supramolecular chain crosslinking. This article highlights some achievements in the synthesis and application of sensitive microgels. In one area of focus, the article discusses the use of sensitive microgels as model colloids to study relations between structure, dynamics, and properties of soft matter. In another area of focus, the paper discusses the use of these microgels to encapsulate, host, and release functional additives. (c) 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014, 52, 435-449