*2.4. CE Metrics and Indicators*

One important step towards CE mainstreaming is the development of suitable indicators that would help measure the state of transition in both absolute and relative/comparative terms [26,65,106–108]. Research on CE metrics and indicators is ongoing at all levels of analysis (micro, meso, macro), with different indicators trying to capture different dimensions of sustainability (environmental, economic, social) and core principles of the CE ('reduce, reuse, recycle, recover, remanufacture, redesign') [109]. Examples of a quantitative analysis of the CE in the European Union concern individual member states [110,111], groups of member states [112,113], regions [114,115], economic sectors [116,117], or all EU member states [118–125].

The recommended indicators measure different aspects of the CE at the company, regional, and national level [126]. Measures proposed by the EU to progress towards a circular economy at the EU and national level are composed of a set of key indicators that cover production and consumption, waste management, secondary raw materials, and competitiveness and innovation [127]. In the typology of the European Environment Agency (EEA), the indicators are divided into five groups: descriptive indicators, performance indicators, efficiency indicators, policy effectiveness indicators, and total welfare indicators [128]. Different methodologies of clustering and classification are proposed, both conceptual and empirical, to deal with the humongous number of available sustainable development indicators (SDI) [65,129–134].
