Various aspects of the cerebral blood-flow regulation can be assessed by transcranial Doppler (TCD). This chapter describes and discusses the approaches that have been reported in the literature. The steady-state characteristics of the cerebral autoregulation can be determined by changing the blood pressure level, and calculating the response of the vasomotors. Moreover, the lower limit of the autoregulatory 'plateau' can be investigated after lowering the blood pressure by pharmacological means. The excellent time-resolution of the TCD technique also facilitates the determination of the quasi-stationary and dynamic aspects of the autoregulatory response. The leg-cuff method shows that regulatory action is very fast, compensating for a sharp drop in blood pressure within seconds. Less intrusively, the autoregulatory characteristics can be assessed from recordings of spontaneous variations in blood pressure. Transfer function methods describe the faster aspects of the mechanism, while correlation techniques reveal the quasi-stationary characteristics. However, the repeatability and accuracy of methods based on spontaneous fluctuations are probably less than those of stimulus-response tests. In this chapter, the various CO2 and acetazolamide approaches that determine vasomotor reactivity are described and discussed.