Erik Winfree is an assistant profes- sor in computer science and com-

and processes, from the storage and reproduction of genetic information to the control of developmental processes to the sophisticated computations performed by the nervous system. Much as human technology uses electronic microprocessors to control electromechanical devices, biological organisms use biochemical circuits to control molecular and chemical events. The engineering and programming of biochemical circuits, in vivo and in vitro, would transform industries that use chemical and nanostructured materials. Although the construction of biochemical circuits has been explored theoretically since the birth of molecular biology, our practical experience with the capabilities and possible programming of biochemical algorithms is still very young. In this paper, I will review a simple form of biochemical algorithm based on the molecular self-assembly of heterogeneous crystals that illustrates some aspects of programming in vitro biochemical systems and their potential applications. There are two complementary perspectives on molecular computation: (1) using the astounding parallelism of chemistry to solve mathematical problems, such as combinatorial search problems; and (2) using biochemical algorithms to direct and control molecular processes, such as complex fabrication tasks. The latter currently appears to be the more promising of the two. Erik Winfree is an assistant profes-