Semiconductor nanocrystals are tiny light-emitting particles with applications in solar energy conversion, optoelectronic devices, molecular and cellular imaging, and ultrasensitive detection. The quantum confinement effect, which occurs due to the small size of these particles, allows researchers to tune their electronic and optical properties by adjusting their size and shape. This effect enables the tuning of light emission across the ultraviolet, visible, near-infrared, and mid-infrared spectral ranges. The unique properties of semiconductor nanocrystals, such as carrier multiplication, single-particle blinking, and spectral diffusion, make them versatile building blocks for complex nanostructures. The article discusses recent advances in understanding the atomic structure and optical properties of semiconductor nanocrystals, as well as new strategies for band gap and electronic wave function engineering. These strategies include alloying, doping, strain-tuning, and band-edge warping, which are crucial for advancing the development of these particles for optoelectronic and biomedical applications. The authors highlight the importance of surface properties and shell passivation in controlling the fluorescence efficiency and stability of nanocrystals. They also explore the unique optical properties of quantum confined structures, such as on-and-off blinking and carrier multiplication, and the impact of strain on band gap engineering and charge carrier wave functions. Looking ahead, the authors expect significant advancements in both fundamental studies and practical applications of semiconductor nanocrystals, including the synthesis of new nanocrystals with unusual structures and properties, and the development of materials with diverse chemical, elastic, and optical properties. They emphasize the importance of engineering nanocrystals for multielectron generation and efficient charge carrier separation in photovoltaic applications, and the need to minimize the overall size of bioconjugated nanocrystals for biomedical applications.Semiconductor nanocrystals are tiny light-emitting particles with applications in solar energy conversion, optoelectronic devices, molecular and cellular imaging, and ultrasensitive detection. The quantum confinement effect, which occurs due to the small size of these particles, allows researchers to tune their electronic and optical properties by adjusting their size and shape. This effect enables the tuning of light emission across the ultraviolet, visible, near-infrared, and mid-infrared spectral ranges. The unique properties of semiconductor nanocrystals, such as carrier multiplication, single-particle blinking, and spectral diffusion, make them versatile building blocks for complex nanostructures. The article discusses recent advances in understanding the atomic structure and optical properties of semiconductor nanocrystals, as well as new strategies for band gap and electronic wave function engineering. These strategies include alloying, doping, strain-tuning, and band-edge warping, which are crucial for advancing the development of these particles for optoelectronic and biomedical applications. The authors highlight the importance of surface properties and shell passivation in controlling the fluorescence efficiency and stability of nanocrystals. They also explore the unique optical properties of quantum confined structures, such as on-and-off blinking and carrier multiplication, and the impact of strain on band gap engineering and charge carrier wave functions. Looking ahead, the authors expect significant advancements in both fundamental studies and practical applications of semiconductor nanocrystals, including the synthesis of new nanocrystals with unusual structures and properties, and the development of materials with diverse chemical, elastic, and optical properties. They emphasize the importance of engineering nanocrystals for multielectron generation and efficient charge carrier separation in photovoltaic applications, and the need to minimize the overall size of bioconjugated nanocrystals for biomedical applications.