Cerebral blood flow (CBF) controls the delivery of glucose and oxygen to the brain. Blood flow is increased to activated tissue in a phenomenon known as functional hyperemia. Currently, either optical techniques or fMRI is utilized to monitor CBF. This presentation will discuss efforts to use electrolytic hydrogen clearance (EHC) to monitor changes in blood flow in the brain. In EHC hydrogen is generated by an electrode and its clearance monitored by a separate electrode. The rate of clearance can be correlated with physical processes that alter its local concentration, such as diffusion and blood flow. In this work, a microfabricated sensor comprised of a pair of platinum electrodes is utilized for making EHC measurements. Microfabricating the sensor enables small probes to be fabricated, small enough to enable use deep in the brain. This sensor has been characterized ex vivo, demonstrating its operation. More recent efforts have transitioned the sensor in vivo to compare blood flow in the cortex during spreading depolarization and in the striatum during pharmacology.