This note explains the methodology used to identify the largest firms in Viet Nam. It also discusses some issues with the data and the efforts made to correct where possible. The annual General Statistics Office (GSO) Enterprise Survey was used to determine the largest firms. The firm list for each year is based on the results of the previous year’s survey plus a list of new enterprises provided by the Ministry of Finance tax office. Coverage is complete for enterprises with more than 10 employees. The unit of analysis is the independent accounting enterprise with its own legal status. Sector of operation is defined as the activity contributing the largest share to enterprise output. If this cannot be determined, then largest share of employment is used (Jammal et al 2006).