Quantum dots (QDs) are nanoscale semiconductor crystals that are typically composed of elements from groups II-VI or III-V of the periodic table. Owing to their small size, ranging from 1 to 10 nm in diameter, coupled with their unique optical and electronic properties, they are pivotal tools in biological applications. The distinctive features of QDs include tunable photoluminescence (PL) emission, high fluorescence quantum yield, robust photostability against bleaching, high extinction coefficients, and fluorescence intermittency. This chapter provides an overview of QD research, including synthesis techniques, photophysical and photochemical analyses, and in vitro and in vivo applications. Even with these developments, problems with toxicity, stability, and scalability still exist. However, using QDs in conjunction with artificial intelligence in the future, QDs are expected to be used in human health diagnosis and treatment.