Light–matter interaction is a key branch of photonics/optical material science. As the research emphasis in recent years has shifted from microscale toward nanoscale, light–matter interaction within extreme dimensions raises new challenges as well as opportunities. However, because of the classic diffraction limit of conventional optics, coupling and confinement of light into deep-subwavelength volume is usually very challenging, resulting in difficulties in exploring the light–matter interaction within ultrathin and ultrasmall dimensions. Based on recent advances in theoretical modeling, nanomanufacturing and experimental validation efforts, unique features have been recognized. Here, recent key progresses of light–matter interaction within extreme dimensions are summarized and future directions based on new combinations of materials, structures, nanomanufacturing, and applications are discussed, ranging from quantum plasmonics, nonlinear optics, to optical biosensing.