The Contingent Worker Supplement (CWS) to the Current Population Survey (CPS), fielded six times between 1995 and 2017, was designed to measure jobs that were temporary in nature as well as work arrangements thought to be associated with less commitment between workers and employers. The latter includes independent contractor and platform work, temporary help and other intermediated contract work arrangements, and on-call work, which captures a certain type of unpredictable work schedule. While the CWS provides consistent measures of the work arrangements covered in the survey over a 22-year time span, it has shortcomings. Data from other household surveys, employer surveys, and administrative records provide important complementary and sometimes conflicting evidence on the alternative work arrangements measured by the CWS. Through a combination of new empirical analysis and a synthesis of existing research findings, we provide insights from these other data sources into the incidence and trends in alternative work arrangements, the characteristics of workers in these arrangements, and the implications of these arrangements for worker outcomes. Our analysis reveals large discrepancies between the CWS and alternate data sources in the size of the independent contractor workforce. In other cases, compared to the CWS, alternate data sources provide considerably broader measures of work arrangements, affecting our understanding of the number and characteristics of workers in them. We discuss lessons from our findings for improving the measurement of alternative work arrangements.