Quantum dots (QDs) have emerged as versatile and powerful tools in the biomedical field, offering unique optical and electronic properties for a wide range of applications. These nanoscale semiconductor particles exhibit narrow, symmetric emission bands, making them particularly suitable for bioimaging, biosensing, and drug delivery. Their size-tunable fluorescence and high photostability allow for precise detection, tracking, and manipulation at the molecular and cellular levels, which are crucial for advanced biomedical applications. QDs enable high-resolution imaging and multianalyte detection, proving invaluable in cancer detection, infection monitoring, and cellular studies. Despite their significant advantages, integrating QDs into clinical and diagnostic settings raises concerns about their potential toxicity and environmental impact. QDs can induce toxicity at the cellular and organ levels, primarily through the release of heavy metal ions and reactive oxygen species (ROS). This chapter introduces QDs, highlighting how their size-tunable fluorescence and high photostability enhance imaging, drug delivery, and biosensing. This work delves into the synthesis and functionalization of QDs and explores how surface modifications and coating techniques can increase their biocompatibility and reduce their toxicity. By addressing both the biomedical applications and potential risks associated with QDs, this chapter provides a comprehensive overview of the current state of QD technology. This finding underscores the transformative potential of QDs in biomedicine while also highlighting the challenges and solutions related to their toxicity and environmental impact.