Sir Harold Himsworth was a renowned clinician and researcher. Among his myriad of accomplishments, he was the first to differentiate diabetes mellitus into 'insulin-sensitive' and 'insulin-insensitive' forms. To help distinguish these two types, he developed the first test to measure insulin sensitivity, the insulin-glucose test. This article reviews Himsworth's pioneer methods and subsequent advances in the measurement of insulin action. The advantages and limitations of commonly used methods to assess insulin sensitivity are also examined.