Fluorescent quantum defects are fluorescent defects in the semiconducting carbon nanotube (SWCNT) hosts. Fluorescent quantum defects allow the chemical modifications on the SWCNT side walls while keeping their fluorescence properties. This enables numerous applications such as sensing and targeting that were not possible or not ideal in the past. For example, SWCNT as in vivo fluorescent probe relied on the non-covalent conjugation of the SWCNTs to the targeting motifs, but these surface coatings were usually displaced by the proteins in the blood stream, hampering the targeting efficacy. The fluorescent quantum defects allow conjugations of these motifs through covalent bonds, thus solving this critical issue. In addition, fluorescent quantum defects are the only room-temperature single photon emitters at telecom wavelength. Single photon source is one of the essential components for photonic-based quantum computing/ communications. In this review, we will describe the fundamental photophysical properties of fluorescent quantum defects, summarize all types of fluorescent quantum defects, list important applications, and illustrate the future of fluorescent quantum defects.