This review article discusses PDMS (polydimethylsiloxane) microfluidic devices and their biological applications. First, the already developed devices are classified from the viewpoints of underlying technology within a common logical framework comprising single-layer, multilayer, and integrated devices, as well as surface chemistry modifications of PDMS. Combinatorial techniques are applied to re-derive existing devices within this framework. Next, the relevant scales of both microfluidics and biology are compared, obtaining the promise and limitations of PDMS microfluidics. Finally, the body of work is reclassified in terms of addressed biological applications and compared to the standard methods in cellular and molecular biology, to offer insights for future devices and applications.