The scanning transmission electron microscope (STEM) was a rediscovery or reinvention whose conception was based on techniques and processes in nuclear physics, and it was lamentably ignorant of any precedent. This chapter is concerns the principles of operation of the STEM, paying particular attention to the limits of performance, it is helpful first to understand the principles behind the formation of images and spectra in the STEM. Axial bright-field imaging is performed in the STEM by placing a small detector on the optic axis beyond the sample. The most commonly used imaging mode in STEM is that using an annular dark-field detector. Spectroscopy is an important application in STEM, with the common ones being energy-dispersive x-ray and electron energy loss spectroscopy. The last few years have seen dramatic advances in the technology of spherical aberration correction for TEM, and commercial correctors are available for both STEM and conventional TEM instruments.