UDEC (Universal Distinct Element Code) is a numerical modeling code for advanced geotechnical analysis of soil, rock, and structural support in two dimensions. UDEC simulates the response of discontinuous media (such as jointed rock) that is subject to either static or dynamic loading. UDEC is a discontinuum code that simulates either the quasi-static or dynamic response to loading of rock media containing multiple, intersecting joint structures. Because it is not limited to a particular type of problem or initial condition, UDEC may be applied to any case where an understanding of the two-dimensional response of such structures is needed. Highlighted Features: Simulation of large displacements (slip and opening) along distinct surfaces in a discontinuous medium (e.g., jointed rock masses); Discontinuous medium treated as an assemblage of discrete (convex or concave) polygonal blocks with rounded corners; Discontinuities treated as boundary conditions between blocks; Relative motion along the discontinuities governed by linear and non-linear force-displacement relations for movement in both the normal and shear directions