In order to utilize Doppler modalities in an optimal manner, one should both be aware of their potential and respect their limitations. To this end, a variety of technical and physical factors affecting the performance of Doppler devices are discussed: (1) the velocity of propagation limiting the temporal resolution of pulsed systems; (2) intervening tissue that may weaken the echo considerably; (3) the size of the sample volume that could be too small to provide a transvascular mean velocity or too large to discriminate flow in adjacent vessels; (4) the principles of continuous wave mode lacking depth resolution, and pulsed mode being prone to aliasing; (5) the additional complexity of duplex/triplex systems; (6) the tight temporal restrictions of 2-dimensional color mappers which cause poor velocity resolution, and (7) the angle of incidence that needs to be assessed in order to measure absolute velocity and the cross-sectional area that must be measured additionally for the calculation of the volume flow rate. Based on this discussion, some hints on selecting instruments or instrument parameters are given.