A system for monitoring biological processes in vivo is provided. The system comprises an implantable luciferase biosensor comprising luciferase in a biocompatible matrix and a caged luciferin probe. The caged luciferin probe can be administered to a living subject and upon encountering a biological activity, e.g., enzyme activity, the caged luciferin probe releases free luciferin which can then interact with the biosensor luciferase to generate light. The light can then be detected and/or measured by a light detector. Compositions, methods and kits related to the system are provided herein.