Diffraction in the polycrystal/crystalline powder is one of the most powerful techniques in study of microstructure and crystal structure of solids. This technique enables, in synergy with microscopic, spectroscopic and other physical techniques, a complete analysis of one-phase and multi-phase substances, that are important in scientific and technological fields. Information on microstructure and crystal structure of a substance is stored in its diffraction pattern ; in order to reveal this information, the diffraction pattern should be decoded by application of adequate mathematical and physical procedures which may often be rather complex. During the last decades, diffraction techniques in the polycrystal are developing rapidly due to the introduction of sophisticated instrumentation, powerful computers and by application of synchrotron radiation. This advance enables the collection and interpretation of diffraction data in a short real time as well as the study of time-resolved dynamic processes in the crystalline substance. Possibilities of diffraction techniques in the polycrystal are concisely described and illustrated by examples of authors’ scientific studies.