This report describes the design and implementation of Motion Description Language, MDL. The purpose of the project was to design a language adapted for describing motion of a rigid body. Motion is specified by translation of a point and orientation of the body relative to some reference orientation. This point does not have to be a point in the rigid body, but its position should be fixed relative to the rigid body. Motion Description Language was implemented with Flex, Bison and C++ on a Sun Workstation running Solaris.