Naturally occurring radon-222 was evaluated for its use in estimating groundwater flow velocities using single-well tests. Investigations were carried out for four different well scenarios, which revealed the advantages and limitations of the approach. On one hand, it was shown that radon is useful as an environmental tracer because of (1) the low costs of the method, (2) the avoidance of any artificial tracer injection into the aquifer, (3) the immediate availability of results, and (4) the need for only a single monitoring well. On the other hand, several potential sources of error were identified, including poor sampling, inadequate hydraulic connection of the well because of a clogged screen, and an unsuitable well diameter resulting in excessively long or short well water residence times. The practical approach is supported by in-depth theoretical considerations. General recommendations are presented concerning the use of radon as an environmental tracer for groundwater flow assessment.