This chapter describes the operational nationalization dataset in detail. The chapter begins by defining state-owned enterprises (SOEs) with de facto control of operations as a measure of operational nationalization. After describing how operational nationalization is measured, the chapter explains the coding and construction of the 187-country, 116-year dataset of national oil companies (NOCs) based on primary and secondary sources of each country’s petroleum history. Only 70 of these countries are major producers, but for completeness the full sample includes all sovereign countries with populations greater than 200,000 in 2000. This chapter includes several brief examples of NOC varieties, cases of NOC reforms and privatizations over time, as well as varieties of nationalization in nonoil sectors like copper, coal, zinc, cobalt, and lithium. The chapter also discusses how the database compares with existing nationalization datasets.