Electron microscopy is an integral toolbox for materials science and engineering enabling detailed analysis of microstructure and composition of materials at different length scales. Conventionally, the failure of engineering material is analysed via ex-situ, i.e., before and after testing under external conditions, e.g., fracture, deformation and/or heat treatment. However, one of the main limitations of post-mortem or ex-situ examination is the issue of causation so that the mechanism might be interpreted in a completely wrong way. Significant efforts have been made by materials scientists and microscopists during the last few decades in instrumentation development that enables the study of dynamic behaviour of materials in real time, i.e., in situ experiments in advanced electron microscopes (TEM and SEM). In situ studies have opened a new area that provides more convincing results and quantitative data for computational modelling. The combination with digital image correlation (DIC) and artificial intelligence (AI) is now growing as a new trend aiming to solve more complicated phenomena in all material systems. This collection is devoted to the recent progress in in-situ testing of all types of materials inside electron microscopes and will focus on quantitative aspects of these techniques as applied to the understanding of materials’ behaviour under varying conditions.