Reliable measurements of the three-dimensional, three-component (3D3C) velocity field in microfluidics are becoming increasingly important for the design and optimization of lab-on-a-chip devices in biochemical applications. Among the many microscale 3D3C velocity field measurement methods, the ${mu}$-DDPIV technique is the most convenient and reliable, but the low seeding rates and low light intensity due to the small pinholes are the major limitations of this technique. The present review demonstrates the principle of the defocusing technique, calibration procedures and particle tracking algorithms to extract the 3D3C velocity vectors. As an example of ${mu}$-DDPIV applications, this paper presents measurements of a complex 3D3C velocity field in chaotic mixers. This technique can be extended to measure the refractive index of an unknown fluid and to a nano/micro hybrid PIV system for multi-scale measurements. Future perspectives on the development of ${mu}$-DDPIV are addressed.