The aim of microfluidic mixing is to achieve a thorough and rapid mixing of multiple samples in micro scale devices. In such devices, sample mixing is essentially achieved by enhancing the diffusion effect between the different species flows. Passive micromixers utilize no energy input other than the pressure head used to drive the fluid flow at a constant rate. Under such conditions, the flow has a laminar characteristic, and hence mixing relies mainly on the induction of a chaotic advection effect to increase the contact surface and contact time between the species flows. Many passive mixers have been proposed to facilitate this task over the past five years. The details of these mixers are briefly described in the following review. Crown Copyright (C) 2015 Published by Elsevier B.V. All rights reserved.