Key Comparisons are international inter-laboratory studies used to establish the degree of equivalence between national measurement standards. These studies, carried out by National Metrology Institutes, are time-consuming, but necessary to facilitate international trade. Since the signing of the Mutual Recognition Arrangement (MRA) in 1999, approximately 120 Key Comparisons in a wide range of metrological areas have been completed and have results posted in the Key Comparison Database (KCDB) maintained by the International Bureau of Weights and Measures (BIPM) in France and in the International Comparisons Database (ICDB) maintained by the National Institute of Standards and Technology (NIST) in the U.S. As with many new standardized procedures, however, the translation of the guidelines for the conduct of Key Comparisons outlined in the MRA from theory to practice has not always been smooth or obvious. Different groups of metrologists working in different areas have interpreted the MRA in different ways. The practicalities of collecting data that support a specific measurement goal from laboratories all over the world has also had varying impact on the decisions made by the scientists who have planned and participated in Key Comparisons. Supported by a large set of completed comparisons from the KCDB and ICDB, an opportunity to study the methods actually being used to conduct Key Comparisons has now arisen. This paper summarizes work on currently completed Key Comparisons and offers recommendations for the design, analysis, and interpretation of future comparisons.