Quantum dots (QDs), discovered by Alexey Ekimov in 1981, are nanoscale semiconductor particles that are typically smaller than 10 nm and are known for their size-dependent optoelectronic properties driven by quantum confinement effects. These effects result in discrete energy levels, enabling control over their optical properties, such as high brightness, quantum yield, photostability, and tunable emission spectra. QDs have revolutionized fields such as biomedicine, optoelectronics, and energy storage, enhancing the performance of solar cells, LEDs, and lithium-ion batteries. This chapter explores the fundamental properties of QDs, including their excitonic behavior and energy level quantization, highlighting their transformative role in advancing nanotechnology and enabling innovations across various industries.