One of the grand goals of scientific endeavour in the 21st century is to probe and control chemical reactions and biological function in real time. Ideally, this requires tempororal resolution of few to tens of femtoseconds and spatial resolution of sub-angstroms. For gas-phase molecules, ultrafast electron diffraction (UED) and laser-induced electron diffraction (LIED) have been shown to be powerful tools where the change of atomic positions in a molecule can be extracted from the measured electron momentum spectra, thus creating a “molecular movie” that reveals the molecular dynamics in real time. In this chapter we address the structure retrieval methods for both UED and LIED, especially for diffraction images created from molecules that are partially aligned. The basic principle of the LIED and a recent experiment observing bond breaking using LIED are also illustrated.