In a molecular crystal, intermolecular interactions will correspond to specific symmetry elements. If one chooses molecules carefully, one can reliably predict specific intermolecular interactions and the corresponding symmetry operations. The symmetry operations of one and two dimensional networks of molecules can be combined to form rod and layer groups respectively. In many cases of chemical interest the sequence of moving from a molecule to a one dimensional array, then on to two and three dimensions corresponds directly to the symmetry combinations leading from the point group to rod group, to layer group and on to the space group. The structures of a number of disubstituted urea derivatives were determined and are used to illustrate these ideas of molecular design.