The combination of synthetic stably branched DNA and sticky ended cohesion has led to the development of structural DNA nanotechnology over the past three decades. Sticky ends on synthetic molecules can be programmed to interact to self-assemble into a variety of geometrical species. Thus, simple branched molecules lead directly to the construction of polyhedra whose edges consist of double helical DNA, and whose vertices correspond to the branch points. Stiff branched motifs must be used to generate self-assembled two-dimensional and three-dimensional periodic lattices of DNA (crystals). DNA has also been used to make a number of nanomechanical devices, including molecules that change their shape, and molecules that can walk or somersault along a DNA sidewalk. Complex mechanical arrangements have been constructed, such as a nanoscale assembly line.