The application of biosensors for near real-time monitoring of key energy and signaling molecules in the intact brain poses major challenges at a number of levels. In this context, amperometric biosensor functionality is impacted by a variety of physicochemical and biological phenomena, including enzyme substrate and co-substrate sensitivity, interference from endogenous electroactive species, fabrication and response consequences of miniaturization for tissue implantation, and biocompatibility issues. Polymer–enzyme composite (PEC) implantable microbiosensors, incorporating a poly-o -phenylenediamine (Po PD) ultrathin permselective barrier, possess a variety of characteristics which make them suitable for long-term in-vivo electrochemical (LIVE) monitoring. This chapter reviews the use of Po PD in this context, and describes a battery of PEC sensitivity and selectivity parameters which allow development of the basic design in a systematic way in order to understand and improve their performance, and to diversify the analyte range of these novel probes of brain function.