How is an economic system defined? Who are the economic agents, and how is a household distinguished from an enterprise? How is the growth of an economy measured? How is a time series defined, and what is the difference between a trend, a cycle and a seasonal variation? This chapter provides an overview of the main concepts and methodological tools necessary to read and analyse the main economic statistics described in Chapter 4. After introducing the main definitions concerning the economic system, we shall analyse the characteristics of the principal economic agents (households, enterprises and public and private institutions) using examples drawn from OECD countries, and then we shall review the main economic aggregates as defined in national accounts, from gross domestic product to national income. The second part of the chapter will be devoted to a brief presentation of index numbers and time series analysis, which are indispensable tools for “reading” economic statistics.