The display of digitized electron microscope images on a computer screen is a crucial first step in the computation of macromolecular structures. It is also essential to be able to visualize the final computed density map in a way that reveals its shape in three dimensions. Ximdisp is an X-windows based, menu-driven computer program that forms the core of the MRC image processing package. Raw electron microscope images, Fourier transforms, and computed density maps may all be displayed in a variety of ways with a choice of colour representations suitable for manuscript illustration purposes. It gives the user full interactive control over its many functions with clear, simple menus, labels, and editable dialogue boxes. Ximdisp plays a part in single-particle analysis with a straightforward particle selection procedure, in processing 2D crystal and electron diffraction data with extended lattice refinement, and in the analysis of helical structures with filament straightening and interactive Fourier transform display of automatically rotated, padded, and floated particles. The role of Ximdisp in all of these analyses and the most effective ways in which it can be used to display images are described.