Microgels are crosslinked soft particles with a three-dimensional network structure that are swollen in a good solvent. They have frequently been termed smart materials since the size, softness, and interaction forces between particles are tunable by external stimuli such as temperature, pH, or magnetic and electric fields. It is this unique feature that has captured the interest of many scientists across a wide range of disciplines. This brief review covers the basic aspects of the relationships between the network structure and gel properties of the thermally sensitive poly(N-isopropylacrylamide) (pNIPAM) microgels including the phase transition process, the internal structure of microgels, and the phase behavior. Additionally, we highlight the impacts of microgels on the biomedical applications, especially in the gene delivery, cell matrix and differentiation of stem cells. (c) 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 2995-3003