The term theranostics is derived from the combination of two words, namely, therapeutics and diagnostics. It is a practical aspect of personalized medicine. It involves using a combination of one radioactive drug to identify (diagnose) and a second radioactive drug to provide therapy in order to treat a tumor. Gastric cancer, also known as stomach cancer, develops due to the formation of malignant cancer cells in the lining of the stomach. The factors such as age, diet, and stomach diseases result in the development of gastric cancer. Some of the common symptoms of this cancer include unexplained weight loss, severe abdominal pain, nausea and vomiting, difficulty in swallowing, etc. It can be diagnosed with the help of imaging tests such as CT or CAT scans, PET scans, MRI, or upper gastrointestinal endoscopy. Theranostics includes molecular targeting and nanotechnologies which are increasingly finding many applications in the treatment of gastric cancer. In this book, the various advancements in the field of theranostics as well as their application for the treatment of gastric cancer have been looked at in detail. For all readers who are interested in this area of study, the case studies included herein will serve as an excellent guide to develop a comprehensive understanding.