Previous chapters describe how price and expenditure data are collected and validated for the international price and volume comparisons of GDP that Eurostat and the OECD make with purchasing power parities (PPPs). This chapter explains how Eurostat and the OECD use the validated data to compute multilateral PPPs for GDP and its component expenditures. The computation has two stages. During the first stage, PPPs are calculated for basic headings. In the second stage, basic heading PPPs are aggregated with GDP expenditures as weights to obtain PPPs for each aggregation level up to and including GDP. Many methods have been developed to calculate and aggregate PPPs1, but the chapter considers only the methods currently employed for Eurostat and OECD comparisons. It should be read in conjunction with Annex V which provides a numerical example to demonstrate how basic heading PPPs are calculated and subsequently aggregated.