Electronic monitoring device (EMD) data are widely used to measure adherence in HIV medication adherence research. EMD data represent an objective measure of adherence and arguably provide more valid data than other methods such as self-reported measures, pill counts, and drug level concentration. Moreover, EMD data are longitudinal, include many measurements, and yield a rich data set. This article illustrates potential pitfalls associated with this measurement technique, including lack of clarity associated with EMD data, and the extent to which adherence outcomes are affected by data management decisions. Recommendations are given regarding what information should be included in publications that report results based on EMD data so as to facilitate comparisons between studies.