  By scanning a fine open-ended coaxial probe above an operating microwave
device, we image local electric fields generated by the device at microwave
frequencies. The probe is sensitive to the electric flux normal to the face of
its center conductor, allowing different components of the field to be imaged
by orienting the probe appropriately. Using a simple model of the microscope,
we are able to interpret the system's output and determine the magnitude of the
electric field at the probe tip. We show images of electric field components
above a copper microstrip transmission line driven at 8 GHz, with a spatial
resolution of approximately 200 $\mu$m.
