In recent years there has been a large number of attempts to estimate the number of platform workers. Lacking a common definition of platform workers, however, these estimates are not comparable across countries and over time. This paper starts to address these issues, by explaining the concept of platform work, reviewing previous attempts to measure platform workers by private agencies and official statistical agencies and examining how different survey questions can affect the understanding of the respondents. The paper also highlights innovative uses of data that provide greater insights into platform workers and concluded with recommendations on how to measure platform workers in the future.