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In order to achieve the stated objectives of key future experiments such as CIT, it will be necessary to maintain plasmas with confinement properties significantly improved over those observed in auxiliary-heated L-mode-type tokamak discharges. This underscores the importance of understanding the physics responsible for the favorable trends exhibited, for example, by ''supershot'' discharges in TFTR and by H-mode-type divertor plasmas. In addressing this general issue, the present paper reports results of theoretical investigations dealing with: the onset conditions of the microinstabilities most likely to influence enhanced confinement behavior; gyrokinetic particle simulations and analytic studies of the saturation and transport properties of the most persistent forms of these instabilities; and detailed comparisons of experimental results with predictions from transport codes using appropriate microinstability-based diffusivities and with local diffusivities from various theoretical models. 19 refs., 2 tabs.