This paper presents the results of experiments performed on single-phase flow in microchannels. Channel depths ranged between 100 and 500 microns, with a W:H ratio greater than 20:1. Water was the primary working fluid in these studies, with the Reynolds numbers (based on the hydraulic diameter of the channel) ranging between 40 and 4000. The experimental setup used allowed measurements of the local pressure drop as well as visualization of the flow patterns within the channel using microscopy. Data presented include friction factors for various channel geometry and surface conditions. We found that the surface roughness tended to increase the laminar friction factor within the channel. Further, for the smoothest channel, as the channel height (or hydraulic diameter) decreased, the laminar friction factor did not depart from the continuum theory predictions. Further, our pressure drop measurements, corroborated with careful flow visualization tests, reveal that the transition Reynolds number is nearly half of what is previously reported for macroscopic channels.