Recently, the use of telephone sampling methods in epidemiology has been sharply increasing. Properly applied, these methods provide powerful tools. Improperly applied, they may produce invalid results. This review covers many points to which the investigator should be alert. An underlying theme is that bias in studies that use telephone sampling can potentially spring from many sources and should be avoided wherever feasible. In epidemiology, there are two main uses of telephone sampling--in general surveys (cross-sectional studies) and in case-control studies. For the former, the principles differ little from those for general surveys in other fields. For the latter, most of the same principles apply, but case-control studies also have their own unique aspects. In this review, several topics receive detailed treatment. Valid combinations of area code and prefix can be found through careful processing of a file that is available commercially. Three options that can be used singly or in any combination provide broadened adaptability for the Mitofsky-Waksberg method of random digit dialing. Bias can be thwarted by certain steps in the interviewing and by weighting. For population-based and then center-based case-control studies, a scheme that samples controls from household censuses and avoids usual problems is offered.