A physical quantity, 'information,' can be defined and must be included in the second law of thermodynamics. This quantity is different from entropy, though closely related to it. Acquisition and use of information is characteristic of most systems, especially those of biological orgin. Treatment of information as a physical quantity permits many types of machines (in the most general sense) to be understood in respect of a thermodynamic function, and as well an investigation of the extent to which the laws governing information are or may limit their behavior. This limitation is most important at the molecular level, such as in the process of DNA and RNA synthesis. However, it may ultimately have a significant impact on all areas of biology including the theory of evolution, which in light of the concept of information can now be addressed in an important new way.