The performance of devices and circuits is advancing at a rapid pace with submicron design ground rules. The requirements to probe the internal nodes of these ultra-fast, -small, and -dense circuits give rise to great challenges for high speed electron-beam testing. In this paper, we review the development of electron beam testing to achieve simultaneously: 5 ps temporal resolution, 0.1 μm spot size and 3mV/√Hz voltage sensitivity. The newly developed instrument, called the Picosecond Photoelectron Scanning Electron Microscope (PPSEM), is capable of measuring the state-of-the-art bipolar, FET circuits and wiring delays. © 1989.