While there is a growing interest in statistical measurement of the social and solidarity economy (SSE), the SSE often remains “under the radar” of national statistics. This entry focuses on the issue of defining the statistical perimeter of the SSE. The first section examines how the statistical definition of the SSE is constructed. The second section exposes the tools that help harmonize SSE statistics across countries. These tools, however, carry different visions of what distinguishes the SSE from other statistical entities, either based mainly on limited profitability or on democratic governance. The third section briefly examines issues surrounding the measurement of the SSE’s contributions. As national accounts are not yet well equipped to measure non-economic dimensions, other methodologies are required to assess the full contribution of the SSE. We conclude by sharing some concerns about the potential effects of measurement on the definition of the SSE.