Carbon quantum dots (CQDs) have gained increasing attention in numerous applications, including fluorescent probes for chemical and biological sensing, disease diagnosis, biomarkers, and bioimaging. CQDs possess size-tunable optical properties with resistance to photobleaching, high photostability, excellent hydrophilicity, biocompatibility, and low toxicity. However, despite these advantages, there are still several challenges that need to be tackled, such as low quantum yields, the high sensitivity of their triplet states to environmental conditions or instrumentation, the fluorescence quenching mechanism, and the creation of CQDs that are sensitive and selective toward specific targets. Surface modification and heteroatom doping are among the most powerful strategies for enhancing the optical properties of CQDs. Rare earth ions/complexes (RE) possess unique optical advantages due to their 4f electron configurations, such as a narrow luminescent band, long luminescent lifetime, and large Stokes shift. They can be combined with CQDs for enhancing their fluorescent properties, leading to improved sensitivity in sensing applications and higher resolution in bioimaging. This chapter first provides an introduction to the CQDs, their general synthetic strategies, and optical properties. Subsequently, a brief overview of the characteristics of rare earth materials is presented. Finally, the fluorescent probes of RE–CQDs hybrid materials and some of their applications in various fields are exemplified and discussed in the following sections.