Errors in medicine, especially medication errors, have long been recognized as a dimension of quality of care and organizational performance. Recently, however, the magnitude of the issue, or its potential impact on cost, quality of care and patient safety have catapulted this issue to the forefront of national debate on the appropriateness of patient care management. There are still fundamental issues associated with the measurement of errors. Should errors that do not cause patient harm receive much attention? Could there be organizational or system issues that predispose to errors? Are there acceptable measurement models that allow comparative analysis and trending of institutional error rate profiles? This paper presents a systematic review of the measurement aspects for errors in medicine, emphasizing the medication errors’ dimension. An indicator-based, epidemiological model of measurement is proposed which will allow a systematic inquiry into the issues of both preventable and non-preventable errors and their potential for patient harm.