In microfluidics, the typical sample volume is in the order of nL, which is incompatible with the common biosample volume in biochemistry and clinical diagnostics (usually ranging from 1 μL to approximately 1 mL). The recently emerged inertial microfluidic technology offers the possibility to process large volume (∼mL) of biosample by well-defined micro-structures. In contrast to conventional microfluidic technologies, where fluid inertia is negligible and flow remains almost within Stokes flow region with very low Reynolds number Re≪1 (Re=ρfU¯¯¯¯fH/μ , where ρ f, Ū f and μ are fluid density, average velocity and dynamic viscosity, respectively, and H is channel hydraulic diameter), inertial microfluidic devices work within an intermediate Reynolds number range (∼1 < Re < ∼100) between Stokes and turbulent regimes. In this intermediate range, both inertia and fluid viscosity are finite, and several intriguing effects appear and form the basis of inertial microfluidics, including (i) inertial migration and (ii) secondary flow. Due to the superior features of high-throughput, simplicity, precise manipulation and low-cost, inertial microfluidics has attracted significant attention from the microfluidic community. Meanwhile, a number of channel designs that focus, concentrate and separate particles and fluids have been explored and demonstrated. In this chapter, we discuss this fascinating technology from three crucial aspects: (1) fundamental mechanism, (2) microchannel designs and (3) applications. From this chapter, we hope that readers can have a clear understanding on the concept of inertial microfluidics, its working mechanism and potential applications.