PURPOSE:  To provide a discussion on the various methods used to estimate adherence to multiple medications (AtMM). Methods currently employed in the AtMM literature will be presented followed by a discussion of pros and cons of the methods and the calculations used to derive the measure of adherence. DESCRIPTION:  In a systematic review undertaken by the ISPOR Medication Adherence Special Interest Group identified 151 relevant studies on AtMM. Of these, 81 studies used a form of Morisky adherence scale and 54 used either the proportion of days covered (PDC) or medication possession ratio (MPR), each with varying methods to calculate AtMM. Other measures were also identified and several studies used multiple measures. This workshop will present the statistical methods and identify the different ways of deriving multiple medication adherence measures. . Discussion will be undertaken of whether the measurements affected the results and if there were measurements that provided more accurate results compared to the others. This will be followed by an audience interaction (15 minutes) on the advantages and disadvantages of using different identified methods and calculations. The concluding period will allow for wrap up, opportunities for questions and answers and examine recommendations for the next steps. Presented by the Medication Adherence and Persistence Special Interest Group.