The number of countries that regularly participate in international large-scale assessments has increased sharply over the past 15 years, with the share of countries participating in the Programme for International Student Assessment growing from one-fifth of countries in 2000 to over one-third of countries in 2015. What accounts for this increase? This paper explores the evidence for three broad explanations: globalization of assessments, increasing technical capacity for conducting assessments, and increased demand for the microeconomic and macroeconomic data from these assessments. Data were compiled from more than 200 countries for this analysis, for six time periods between 2000 and 2015, yielding more than 1,200 observations. The data cover each country's participation in each of six cycles of PISA as it relates to the country's level of economic development, region, prior experience with assessment, and OECD membership. The results indicate that the odds of participation in PISA are markedly higher for OECD member countries, countries in the Europe and Central Asia region, high- and upper-middle-income countries, and countries with previous national and international assessment experience; the paper also finds that regional assessment experience is unrelated to PISA participation.