Nanocrystals have been established as an important cancer diagnosis and therapeutic tool due to their ability to deliver poorly soluble drugs with sustained release, low toxicity, and flexibility of administration. In this book, the importance and current status of nanocrystals in cancer therapy are presented, along with challenges, and future prospects. The book focuses on several therapies of nanocrystals, including anticancer, immunotherapy, radiotherapy, biotheranostics, targeted therapy, photothermal, and photodynamic. Further, different imaging and diagnostics using nanocrystals are mentioned, including magnetic resonance imaging (MRI), computed tomography (CT), biosensing, and luminescence. Finally, the limitations and potential solutions of nanocrystals in the field of cancer theranostics are also discussed.