Poly (N-isopropylacrylamide) (pNIPAm)-based hydrogels and hydrogel particles (microgels) have been extensively studied since their discovery and "popularization" a few decades ago. While their uses seem to have no bounds, this Feature Article is focused on their development and application for sensing small molecules, macromolecules, and biomolecules. Hydrogel/microgel-based photonic materials with order in one, two, or three dimensions are highlighted, which exhibit optical properties that depend on the presence and concentration of various analytes.