Transmission electron microscopy (TEM) has seen major breakthroughs over the past decade in imaging and spectroscopy at the single atomic level. Spatial resolutions have reached 0.5 Å while energy resolutions have reached 10 meV. In recent years, the focus has shifted towards in situ TEM to probe functionality of materials and to reveal how materials change dynamically under real operation conditions. Recent scientific breakthroughs using in situ TEM have largely benefited from advances in instrumentation. Advances that are specific to enable in situ capabilities include liquid or gas environmental TEM, ultrafast dynamic electron microscopy, quantitative mechanical tests, and electric and magnetic coupling. This MRS Webinar (presented by MRS Bulletin) on in situ TEM will present reviews of specific sub-areas covering the most exciting developments in the field. The talks in the webinar will provide a sampling of how in situ TEM is having a major impact on materials science today.