Ultrasound contrast agents (UCAs) are widely used in Doppler studies, either for simple echo enhancement purposes, or to increase the low signal-to-clutter ratio typical of microcirculation investigations. Common to all Doppler techniques, which are briefly reviewed in this paper, is the basic assumption that possible phase and amplitude changes in received echoes are only associated with UCA microbubble movements due to the drag force of blood. Actually, when UCAs are insonified, phenomena such as rupture, displacement due to radiation force, and acoustically driven deflation might influence the results of Doppler investigations. In this paper, we investigate the possible Doppler effects of such phenomena by means of a numerical simulation model and a special acousto-optical set-up which allows analysis of the behavior of individual microbubbles over relatively long time intervals. It is thus found that all phenomena produce evident Doppler effects in vitro, but that bubble displacement and deflation in particular, are not expected to significantly interfere with clinical measurements in standard conditions.