Semiconductor nanocrystals, or quantum dots, are tiny light-emitting particles on the nanometer scale with unique optical and electronic properties. These properties arise from the quantum confinement effect, which confines electronic charge carriers within the nanocrystal, allowing precise tuning of energy levels and optical transitions by adjusting the size and shape of the particles. This tunability enables light emission across the ultraviolet, visible, near-infrared, and mid-infrared spectral ranges. Semiconductor nanocrystals bridge the gap between small molecules and bulk crystals, exhibiting properties such as carrier multiplication, single-particle blinking, and spectral diffusion. They serve as versatile building blocks for complex nanostructures like superlattices and multimodal agents for molecular imaging and targeted therapy. Recent advances in understanding the atomic structure and optical properties of semiconductor nanocrystals have focused on band gap and electronic wave function engineering. Techniques such as alloying, doping, strain-tuning, and band-edge warping are key to controlling charge carrier locations. These methods enable the development of nanocrystals for optoelectronic and biomedical applications. The quantum confinement effect leads to size-dependent band gaps, with smaller nanocrystals having larger band gaps. The shape of nanocrystals also influences their optical properties, with quantum dots, wires, and wells exhibiting different band gap shifts. Surface properties are crucial, as surface atoms and facets can affect optical behavior. Surface passivation with inorganic shells helps stabilize and enhance fluorescence by reducing surface defects and trap states. Optical properties of semiconductor nanocrystals include unique phenomena like blinking, where single quantum dots intermittently emit light. This blinking is attributed to charge carrier trapping or Auger recombination. Carrier multiplication, where multiple excitons are generated from a single photon, enhances photovoltaic efficiency. Type-II quantum dots, where electrons and holes are localized in different regions, offer benefits for photovoltaic devices due to directional charge transport. Strain tuning modifies electronic band gaps by altering bond lengths, affecting charge carrier wave functions and recombination probabilities. Strain can be used to control optical and electronic properties, with strain-induced band warping influencing fluorescence efficiency. Future research aims to advance both fundamental studies and practical applications of semiconductor nanocrystals. Innovations in synthesis, surface modification, and strain engineering will enhance their use in optoelectronics, biomedical imaging, and photovoltaics. Challenges include minimizing toxicity, improving stability, and optimizing charge carrier separation for efficient energy conversion.Semiconductor nanocrystals, or quantum dots, are tiny light-emitting particles on the nanometer scale with unique optical and electronic properties. These properties arise from the quantum confinement effect, which confines electronic charge carriers within the nanocrystal, allowing precise tuning of energy levels and optical transitions by adjusting the size and shape of the particles. This tunability enables light emission across the ultraviolet, visible, near-infrared, and mid-infrared spectral ranges. Semiconductor nanocrystals bridge the gap between small molecules and bulk crystals, exhibiting properties such as carrier multiplication, single-particle blinking, and spectral diffusion. They serve as versatile building blocks for complex nanostructures like superlattices and multimodal agents for molecular imaging and targeted therapy. Recent advances in understanding the atomic structure and optical properties of semiconductor nanocrystals have focused on band gap and electronic wave function engineering. Techniques such as alloying, doping, strain-tuning, and band-edge warping are key to controlling charge carrier locations. These methods enable the development of nanocrystals for optoelectronic and biomedical applications. The quantum confinement effect leads to size-dependent band gaps, with smaller nanocrystals having larger band gaps. The shape of nanocrystals also influences their optical properties, with quantum dots, wires, and wells exhibiting different band gap shifts. Surface properties are crucial, as surface atoms and facets can affect optical behavior. Surface passivation with inorganic shells helps stabilize and enhance fluorescence by reducing surface defects and trap states. Optical properties of semiconductor nanocrystals include unique phenomena like blinking, where single quantum dots intermittently emit light. This blinking is attributed to charge carrier trapping or Auger recombination. Carrier multiplication, where multiple excitons are generated from a single photon, enhances photovoltaic efficiency. Type-II quantum dots, where electrons and holes are localized in different regions, offer benefits for photovoltaic devices due to directional charge transport. Strain tuning modifies electronic band gaps by altering bond lengths, affecting charge carrier wave functions and recombination probabilities. Strain can be used to control optical and electronic properties, with strain-induced band warping influencing fluorescence efficiency. Future research aims to advance both fundamental studies and practical applications of semiconductor nanocrystals. Innovations in synthesis, surface modification, and strain engineering will enhance their use in optoelectronics, biomedical imaging, and photovoltaics. Challenges include minimizing toxicity, improving stability, and optimizing charge carrier separation for efficient energy conversion.