A few comments about the defects of ideal crystals are made on the occasion of the international year of crystallography that penetrates into many branches of science. This year various events organized by the international as well as national crystallographic societies took place worldwide. A history of particular discoveries originated in the French Friedel‘s family concerning the twin boundaries, dislocations and terminology of liquid crystals is also described in the article., Václav Paidar., and Obsahuje seznam literatury