Continuous wave (CW) Doppler ultrasound is widely used for the diagnosis of different vascular diseases. A CW Doppler is a device that allows examining blood flow and blood flow velocity by implementing the Doppler effect. CW Doppler devices are portable and simple to use. CW Doppler is used mostly to provide valuable first-line diagnostic evidence as to the presence, site, and severity of peripheral arterial disease (PAD), by determining the ankle-brachial index (ABI). The measurement of ABI is now the first step in the objective diagnosis of PAD. The ABI is also an indicator of atherosclerosis at other vascular sites and is accepted as a prognostic marker for cardiovascular events in asymptomatic subjects with PAD. Velocity waveform analysis can be useful to characterize PAD anatomy and, in case of incompressible crural arteries, it can confirm the presence of PAD. The shape of the arterial velocity waveform is characteristic to individual arteries and to specific sites along the arteries. CW Doppler was widely used for the investigation of the venous flow in different disorders, like deep vein thrombosis (DVT) and venous reflux. Continuous wave Doppler cannot identify damage of the walls, the presence of thrombus or anatomical variations of the veins; therefore, in the diagnoses of DVT, it was replaced by duplex ultrasound. Nevertheless it can be used as a screening investigation to identify venous reflux.