Many researchers have demonstrated microfluidic devices based on magnetic forces; however, the widespread deployment of magnetic microfluidic devices has been slowed by issues of difficult microfabrication and integration of magnetic materials with polymer materials that are often employed for microfluidics. Relatively new advances in magnetic composite polymers (M-CPs) may offer solutions to these problems. Polymer materials have been demonstrated that can be micropatterned using conventional microfabrication methods such as photolithography or micromolding, but are rendered magnetic through the introduction of micro- or nanoparticles into the polymer matrix. These materials retain many of the matrix polymer's characteristics and remain compatible with their un-doped polymer base. Softly-magnetic nanocomposite polymers are sufficient for many applications, such as unidirectional actuators involving microstructure attraction. However, hard-magnetic composite polymers can be permanently polarized, and find applications in areas such as hi-directional actuation or bead capture and orientation. This critical review provides an introduction to magnetics as applied to M-CPs, and discusses the application of M-CPs to various microfluidic devices and applications, including fluid manipulation devices; interconnects and assembly aids; and movable devices for, e.g., cell-culture. We further discuss the potential outlook for magnetic-composite-polymer-driven technological advances in the fields of microfluidics and labs-on-a-chip (LOC). (C) 2014 The Electrochemical Society. All rights reserved.