Microscopic particle image velocimetry (microPIV) experiments were performed on smooth and roughened microchannels. These measurements represent the first instantaneous velocity field measurements in a turbulent microscale flow;The effect of hydraulic diameter on flow through smooth square polydimethylsiloxane (PDMS) microchannels was investigated by varying the hydraulic diameter from 200 mum to 640 mum for Reynolds numbers ranging from 200 through 3971. The data was analyzed to yield mean velocity profiles u, velocity fluctuations (u') and (v'), and Reynolds stresses - ( u'v') and these were compared to experimental data of macroscale channel flow. The microscale and macroscale results agreed quite well, indicating there are no significant differences between microscale and macroscale channel flow;Next, the effect of aspect ratio on flow through smooth rectangular microchannels was investigated by varying the aspect ratio from 0.97 to 5.69 for Reynolds numbers ranging from 200 through 3267. By analyzing mean velocity profiles u, velocity fluctuations (u') and (v'), and Reynolds stresses---(u'v') and comparing these experimental data with macroscale results, the data were used to clarify the discrepancies on transition in microfluidic flow due to the aspect ratio effect that was observed by previous researchers;The statistical analysis of spatial correlation coefficients of velocity fluctuations to characterize large scale turbulent structures in these two experiments indicated that the large scale turbulent structures observed in microchannels are similar to their macroscale counterparts;Flow within roughened microchannels of dimensions 600 mum x 400 mum with one single microstructure measuring 120 mum x 120 mum x 43 mum was studied by analyzing the velocity profiles in the wakes of microstructures. The data were obtained for comparison of the experimental velocities with CFD results at varying downstream locations and depths to provide for the validation of CFD in microfluidic flow study;Finally, the experiments were performed in the roughened microchannels 600 mum x 400 mum with one single cubic microstructure dimensioning at 20 mum 40 mum and 50 mum for Reynolds number ranging from laminar to turbulent. The data obtained at upstream and various downstream locations and various depths indicated significant turbulence enhancement by introducing man-made surface roughness in microchannels.