Cancer‐targeted nanotechnology is experiencing the trend of finding new materials with multiple functions for imaging and therapeutic applications. With the rapid development of the related fields, there exists a large number of reports regarding theranostic nanomedicine, decreasing the gap between cancer diagnosis and treatment with minimized separate comprehensions. In order to present an overview on the cancer‐targeted nanotheranostics, we first describe their essential building blocks, including platforms, therapeutic agents and imaging agents, and then the recently rapidly developed multimodal theranostic systems. Finally we discuss the major challenges and the perspectives of future development of nanotheranostics toward clinical translations and personalized nanomedicine. Essential segments of theranostic nanomedicine are reviewed, including platforms, therapeutic options and imaging modalities, and then examples of typical nanotheranostic systems, with a particular focus on recently developed multimodal theranostics for optimal cancer therapy. Finally, the major challenges and possible future directions in the field are discussed.