The forced oscillation technique (FOT) is another application of the pressure oscillation method for lung diagnostics. It is based on the measurement of the respiratory impedance, which has evolved into powerful tools for assessing various mechanical phenomena in the mammalian lung during health and disease. Although FOT was introduced in the previous chapter, it is presented in detail in this chapter, including working principle, instrumentation, measurement arrangement, and impedance measurement methods. Finally, some clinical applications of FOT are also described.