In the field of DNA nanotechnology, synthetic DNA strands are used to build molecular systems that can exhibit programmable functions of increasing complexity. The unique predictability of DNA-DNA interactions allows rationally designed synthetic DNA strands to be used as building blocks for the construction of 2D and 3D nanostructures with defined geometries. Synthetic DNA strands can also be site-specifically modified with a wide range of functional components, including optical labels, recognition elements, and proteins, giving otherwise inert DNA-based structures different functionalities. In addition, DNA switches and devices can also be designed to respond to external stimuli such as temperature or pH. The programmability and responsiveness of synthetic DNA described above has been exploited over the past two decades to create DNA-based structures that can perform mechanical work in response to specific inputs. Recently, basic machine elements made of DNA such as hinges, joints, arms, and levers have been demonstrated that can be controlled by DNA-DNA interactions or by environmental stimuli such as pH, ionic strength, temperature, or external fields. Recently, for example, Dietz and Simmel described a rotating ratchet motor made entirely of synthetic DNA. The rotor achieves rotational speeds and torques similar to those of other natural molecular machines, such as ATP synthase, and can move in a specific direction by applying a simple external AC field. DNA-based devices and machines can also be developed for molecular transport mechanisms.