What do scanning electron microscopes and transmission electron microscopes have in common

 Both require a vacuum.

The transmission electron microscope and scanning electron microscopes are two microscopic techniques which uses a high speed and energy electron beams to create an image of the specimen. Scanning electron microscopy creates the image of the specimen by detecting reflected electrons from specimen whereas transmission electron microscopy involves transmission of electrons by the specimen. Both scanning and transmission electron microscopes require vacuum. The flow of electrons is facilitated by a chamber maintained at high vacuum. The vacuum increases the speed and energy of electrons interacting with the specimen.