XCrySDen is a molecular and crystalline-structure visualization program, but its main function is as a property analyzer program. It can run on most UNIX platforms, without any special hardware or software requirements. Special efforts were made to allow for appropriate display of 3D isosurfaces and 2D contours, which can be superimposed on crystalline structure and interactively rotated and manipulated. XCrySDen is also a graphical user interface for the CRYSTAL95/98 (Saunders, V. R., et al. CRYSTAL98--User's Manual. University of Torino, Turin, Italy, 1999) ab initio program and a visualization system for the WIEN97 (Blaha, P., et al. Comput. Phys. Commun. 1990, 59, 399) ab initio program. In this article the program functions are presented with a short description of the algorithms.