Flow visualizations are performed to study the hydrodynamic characteristics of laminar flow in tortuous microchannels for a wide range of Reynolds numbers. The detailed flow patterns in wavy channels are identified and found to be greatly affected by geometrical parameters. In wavy channels, steady flow exists at low Reynolds numbers, but above a critical number unsteady flow develops. The critical Reynolds number is found to depend on the characteristics of the channel path. Recirculation zones form immediately after the inner corners of the sharp bends, with their size and magnitude of the recirculation velocity increasing with higher Reynolds numbers. Large fluctuations in recirculation zone locations highlight the importance of these flow features in the development of transient flow. The flow behaviors play very important roles in determining the pressure drop in wavy channels relative to straight channels.