Public authorities are major consumers in Europe: they spend approximately 2 trillion euros annually. By using their purchasing power to choose goods and services with lower impacts on the environment, they can make an important contribution to sustainable consumption and production. Environmentally friendly (green) purchasing is also about influencing the market. Green Public Procurement is a voluntary instrument, which means that individual Member States and public authorities can determine the extent to which they implement it. SIGMA Brief 13 offers guidance for Contracting Authorities on what can be considered as environmental considerations (e.g. increasing the energy efficiency of buildings, encouraging the development of alternative energy sources) and how these considerations can be incorporated into the procurement process in line with EU law.