A wide variety of magnetically confined plasmas, including many tokamaks suchas the JET, TFTR, JT-60U, DIII-D, RTP, show clear evidence for the existenceof the so-called `internal transport barriers' (ITBs) which are regions ofrelatively good confinement, associated with substantial gradients intemperature and/or density. A computational approach to investigating theproperties of tokamak plasma turbulence and transport is developed. Thisapproach is based on the evolution of global, two-fluid, nonlinear,electromagnetic plasma equations of motion with specified sources. In thispaper, the computational model is applied to the problem of determining thenature and physical characteristics of barrier phenomena, with particularreference to RTP (electron-cyclotron resonance heated) and JET (neutral beamheated) observations of ITBs. The simulations capture features associated withthe formation of these ITBs, and qualitativelyreproduce some of the observations made on RTP and JET. The picture of plasmaturbulence suggested involves variations of temperature and density profilesinduced by the electromagnetic fluctuations, on length scales intermediatebetween the system size and the ion Larmor radius, and time scalesintermediate between the confinement time and the Alfvén time(collectively termed `mesoscales'). The back-reaction of such profile`corrugations' (features exhibiting relatively high local spatial gradientsand rapid time variations) on the development and saturation of the turbulenceitself plays a key role in the nonlinear dynamics of the system. Thecorrugations are found to modify the dynamical evolution of radial electricfield shear and the bootstrap current density, which in turn influence theturbulence. The interaction is mediated by relatively long wavelength,electromagnetic modes excited by an inverse cascade and involving nonlinearinstabilities and relaxation phenomena such as intermittency and internal mode locking.