The performance of devices and circuits is advancing at a rapid pace with the advent of submicron design ground rules and sub-50 ps switching times. The requirements to probe the internal nodes of these ultrafast, ultrasmall, and ultradense circuits give rise to great challenges for high-speed electron-beam testing. In this paper, we review the steps which have allowed electron beam testing to achieve simultaneously: 5 ps temporal resolution, 0·1 μm spot size and 3 mV/ (formula presented)Hz voltage sensitivity. The resulting newly developed instrument, called the picosecond photoelectron scanning electron microscope (PPSEM), is capable of measuring the state-of-the-art bipolar and FET circuits, as shown in this paper. © 1988 Taylor & Francis Group, LLC.