Plasma turbulence enhances the diffusion of heat, momentum and particles across magnetic surfaces. As a result, a central focus of scientific research within the magnetic confinement fusion program has been to measure, understand and control turbulence in high temperature plasma. Since 1988, DOE's ``Transport Task Force'' (TTF) initiative has fostered and stimulated substantial and wide-ranging research in this area. Since 1991, DOE has incubated algorithm development and code development in gyrokinetics, with outstanding results. The successes in simulating plasma turbulence have been based on more than increasing CPU power. Algorithmic and theoretical advances were also required to simulate the extremely anisotropic dynamics and the wide range of time scales, to understand the simulation results, and to construct reduced models to allow meaningful experimental tests and comparisons.

The most important success of gyrokinetic and gyrofluid simulations is the identification of ion-temperature-gradient-driven (ITG) instabilities as a dominant source of tokamak turbulence and transport. Indirect evidence for the important role of ITG includes: numerical agreement between calculated and measured fluxes, basic features of fluctuation spectra, the importance of edge/pedestal performance to overall performance, the ``stiffness'' of ion temperature profiles at high temperatures, many of the general trends seen in the dependence of energy transport on engineering and physics parameters, the influence of sheared flows on the energy transport and transport barriers, and the importance of zonal flows generated by turbulence. However, direct evidence - in the form of simultaneous measurements of profiles and fluctuations which agree with first-principles, direct numerical simulations - is lacking. The two principal difficulties have been:

• The lack of well-resolved, self-consistent experimental datasets, including flows, impurity density profiles, and fluctuation measurements, and
• Generally inadequate physics content in simulations: full geometry, impurities, flows, and electron dynamics are all required.

Experiments are, however, improving rapidly and self-consistent datasets are being produced by the major devices. NSTX and DIII-D experimental facilities are prepared to measure turbulent fluctuations in fully characterized discharges. The codes now contain essentially all the relevant physics and are benchmarked and ready for detailed comparison with data.