<p>Nowadays laser-plasma-based wakefield accelerators are delivering high quality electron beams of relevance for emerging applications in medicine, industry, and fundamental science. Many of these applications critically require the precise characterization of the accelerated electron bunch as well as the plasma wakefield that largely affects the bunch’s quality. Advanced diagnostics of such highly transient, microscopic bunch and field structures, however, remains very challenging. In this review, we present an overview of a novel technique, termed femtosecond ultrarelativistic electron microscopy (FREM), designed to address this challenge. FREM utilizes a high-energy electron bunch, generated by a separate laser-plasma accelerator, as a probe. This single-shot FREM technique has been applied to characterize several important processes existing in plasma wakefield, including the fine structure of linear and highly nonlinear plasma waves, accelerated electron beam structure and its evolution, the transition from laser driven wakefield to electron bunch driven wakefield and a tilted shock front induced wakefield splitting, all with very high spatiotemporal resolution. Together with all these results, FREM is expected to significantly advance the understanding of the complex laser-beam-plasma dynamics for the real-time optimization of plasma accelerators.</p>