Objectives: Once a core outcome set (COS) has been defined, it is important to achieve consensus on how these outcomes should be measured. The aims of this systematic review were to gain insight into the methods used to select outcome measurement instruments and to determine whether methods have improved following the COnsensus-based Standards for the selection of health Measurement INstruments (COSMIN)/Core Outcome Measures in Effectiveness Trials (COMET) guideline publication. Study Design and Setting: Eligible articles, which were identified from the annual COMET systematic review, concerned any COS development studies that provided a recommendation on how to measure the outcomes included in the COS. Data were extracted on the methods used to select outcome measurement instruments in accordance with the COSMIN/COMET guideline. Results: Of the 118 studies included in the review, 48% used more than one source of information when finding outcome measurement instruments, and 74% performed some form of quality assessment of the measurement instruments. Twenty-three studies recommended one single instrument for each core outcome included in the COS. Clinical experts and public representatives were involved in selecting instruments in 62% and 28% of studies, respectively. Conclusion: Methods used to select outcome measurement instruments have improved since the publication of the COSMIN/COMET guideline. Going forward, COS developers should ensure that recommended outcome measurement instruments have sufficient content validity. In addition, COS developers should recommend one instrument for each core outcome to contribute to the overarching goal of uniformity in outcome reporting.