This monograph presents the microscales of complex (buoyant, thermocapillary, two-phase, reacting, radiating, pulsating, etc.) turbulent flows and interprets heat and mass transfer correlations in terms of these scales. The author introduces a general methodology for the development of microscales for complex turbulent flows. Then he provides, by these scales, a fundamental interpretation for a number of momentum, heat, and mass transfer correlations which are assumed to be empirical. Lastly, he develops correlations in terms of these scales for environmentally and/or technologically important problems related to buoyancy driven flows, pulsating flows, diffusion flows, fires, etc.