turbulence, state of violent or agitated behavior in a fluid. Turbulent behavior is characteristic of systems of large numbers of particles, and its unpredictability and randomness has long thwarted attempts to fully understand it, even with such powerful tools as statistical mechanics. Although much is still unknown about turbulence, recent developments in nonlinear dynamics have led to an understanding of the onset of turbulence, and the advent of the supercomputer has enabled better models of turbulent states to be developed. Until the early 1970s, it was held that laminar, or smooth, flow made a gradual transition to turbulent flow by the addition of instabilities, one at a time, until the flow became unpredictable. Experimental work, however, has shown that the onset of turbulence occurs abruptly, and in fact is characterized by the so-called strange attractors of nonlinear dynamics. Increased understanding of turbulent flow through supercomputer models is leading to advances in such diverse areas as the design of better airplane wings and artificial heart valves.