Electron crystallography has a storied history which rivals that of its more established X-ray-enabled counterpart. Recent advances in data collection and analysis have sparked a renaissance in the field, opening a new chapter for this venerable technique. Burgeoning interest in electron crystallography has spawned innovative methods described by various interchangeable labels (3D ED, MicroED, cRED, etc.). This Review covers concepts and findings relevant to the practicing crystallographer, with an emphasis on experiments aimed at using electron diffraction to elucidate the atomic structure of threedimensional molecular crystals. CONTENTS 1. Introduction and Historical Background 13883 2. Theoretical Foundations 13884 2.1. Differences Between X-ray and Electron Scattering 13884 2.2. Differences Between X-ray and Electron Wavelengths 13886 2.3.