Quantum dots are semiconductor nanomaterials with exceptional optoelectronic properties showing significant potential in cancer diagnostics and treatments. Known for their fluorescence intensity, stability, and highly tunable emission, QDs are of considerable importance for their ability to image and target cancer cells at both in vitro and in vivo levels. This chapter explores the application of QDs in bioimaging, biosensing, and theranostics, with a focus on photodynamic therapy and the development of biocompatible QDs for real-time imaging in cancer research and treatments. QDs are critical in facilitating visualization of cellular uptake, cancer cell dynamics, and receptor profiling, which are crucial for early detection of the disease. QDs have also been employed for targeted drug delivery, where therapeutic molecules have been transported to the cancerous cells by surface-functionalized QDs with minimal impact on healthy tissues. The chapter also focuses on the challenges regarding QD toxicity and hence emphasizes the green surface-modified synthesis of QDs for reduced toxicity, enabling safe and biocompatible applications in cancer research and theranostics.