A wide variety of chemotherapy and radiotherapy agents are available for treating cancer, but a critical challenge is to deliver these agents locally to cancer cells and tumors while minimizing side effects from systemic delivery. Nanomedicine uses nanoparticles with diameters in the range of ∼1-100 nm to encapsulate drugs and target them to tumors. The nanoparticle enhances local drug delivery efficiency to the tumors via entrapment in leaky tumor vasculature, molecular targeting to cells expressing cancer biomarkers, and/or magnetic targeting. In addition, the localization can be enhanced using triggered release in tumors via chemical, thermal, or optical signals. In order to optimize these nanoparticle drug delivery strategies, it is important to be able to image where the nanoparticles distribute and how rapidly they release their drug payloads. This Review aims to evaluate the current state of nanotechnology platforms for cancer theranostics (therapeutic and diagnostic particles) that are capable of noninvasive measurement of release kinetics.