This 51st Methodological Note presents the methodology and technical decisions behind the global Multidimensional Poverty Index (MPI) 2021 and the results presented in global MPI 2021 Data Tables 1–8. This document is part of OPHI’s Methodological Notes series. A Methodological Note is published for every release of the global MPI. The global MPI 2021 covers the computation of MPI and its partial indices for 109 developing countries. The estimates are further disaggregated by age groups, rural/urban areas, and subnational regions. A new element in this 2021 round is that we have disaggregated by gender of household head (female and male). In addition, harmonised over time estimates are produced for 84 of the 109 countries. 56 of these countries have harmonised estimates for over two time periods, while 28 countries have harmonised estimates for over three time periods.