Obtaining rapid mixing in microfluidic systems is a problem that must continue to be addressed if microelectromechanical systems are to attain their full potential in commercial markets.We present the paradigm of “designing for chaos” as a general framework for enhancing mixing in microfluidic applications.Designing for chaos is based on a fundamental understanding of the kinematics underlying the mixing process, freeing the MEMS researcher to work with design guidelines instead of empirically determined physical configurations.We have applied this strategy in designing a passive in-line micromixer that relies on three degrees of freedom to create chaos.T he mixer design was fabricated using a compression micromolding process to create threedimensional flow channels in polydimethylsiloxane (PDMS).Computational and experimental analyses demonstrate the effectiveness of the resulting design in generating chaos in the flow and hence enhancing mixing.