GAP package Polyhedral. The package polyhedral is designed to be used for doing all kinds of computations related to polytopes and use their symmetry groups in the course of the computation. The package Polyhedral is devoted to polytopes and lattices. Its main functionalities are: Computing dual description of polytopes by using symmetries for reducing the size of the computation. Compute the automorphism group of a polytope. Compute the volume of a polytope. Compute the K-skeleton of a polytope. Compute the Wythoff construction of a polytope. Compute the Delaunay tesselation corresponding to a lattice of Rn. Deal with L-type domains, i.e. spaces of lattices. Compute with unimodular vector systems. Recognize affine and spherical Coxeter Dynkin diagrams. Enumerate perfect forms in T-spaces.