Results are presented of direct numerical simulations (DNS) of an unsteady, three-dimensional, temporally developing, compressible mixing layer under both non-reacting and reacting non-premixed conditions. In the reacting case, a simple chemistry model of the type A + rB -»(1 + r)Products is considered. Based on simulated results, it is shown that at sufficiently large Reynolds numbers the global and statistical features of mixing transitions are similar to those observed experimentally. At sufficiently large Mach numbers, it is shown that eddy shockletsdo indeed exist in three-dimensional (3D) flow. However, the strength of these shocks is less than that in two-dimensional (2D) layers of the same compressibility level. Aided by the analysis of the DNS data, the extent of validity of the “Steady Laminar, Diffusion Flamelet Model” (SLDFM) and the “Conditional Moment Method“ (CMM) are assessed. In the evaluation of the SLDFM, DNS results for different stoichiometric coefficients and reaction types are analyzed. It is shown that DNS results compare well with model predictions as the magnitude of the Damkohler number is increased. The agreement is improved as the value of r is increased and also as the effects of exothermicity become more pronounced. In the assessment of the CMM, it is shown that the conditional reaction rate can be reasonably approximated in terms of the conditional averages of the scalar variables. Also, the cross-stream variation of the conditional scalar mean values is negligible. However, this is not the case for the variation of higher order moments of the scalar variables.