Two key measures of capital stock exist. The first is productive capital stock, which looks at capital in its function as a provider of capital services in production. The second is gross (or net) capital stock, which captures the role of capital as a store of wealth.1 This Annex provides supplementary information on these two measures and the approaches used to estimate them. It also provides further information on data availability for capital at the OECD and provides some pointers to future developments at the OECD.