Kinetic models have proved useful in describing the behavior of molecular machines, but such analysis is inherently unable to give an account of free energy transfer from the fuel to the machine. The consideration of thermodynamics is essential to understand free energy transduction, how this drives directional motion and generates the capacity to perform work , and, hence, how to optimize motor's design. Up to now attempts to design molecular motors have been led by chemical intuition, with little opportunity to reliably judge the effectiveness of a machine's design or performance unless and until it has been realized experimentally.