The 17 Sustainable Development Goals (SDGs) of the 2030 Agenda comprise 169 targets. These are monitored through hundreds of indicators produced by different institutions at the global, European and national levels. How can such complexity be measured, and how should the results be interpreted? The module presents the logic of monitoring complex, multidimensional phenomena. It starts from the main monitoring systems (United Nations, Eurostat, Istat, ASviS) and moves on to the construction and evaluation of composite indicators. Particular attention is paid to the challenges of data collection, harmonisation and aggregation. It also covers data disaggregation by gender, age and migration background, in line with the principle of "leaving no one behind", and the measurement of progress towards targets. Through examples, comparisons across countries and territories, and exercises with open data, the module provides the tools to answer questions such as: "Which countries are closest to the goal? Which are progressing at the pace needed to reach it by 2030? Who is being left behind within each country?"