This article deals with one of the priorities set by the Rio Summit (1992) within 'Agenda 21' and, in particular, School Agenda 21, whose main aim is to 'green' schools. The overall objective of this study is to evaluate and compare different dimensions relevant to this aim in a sample of 31 Chilean and Spanish eco-schools, ie schools with institutional environmental certification from two provinces of both countries. The data collection instrument was an online survey concerning the following aspects: 1) institutional expansion and projection of the program in Spain and Chile, 2) environmental policy, 3) environmental committee, 4) school leadership, 5) evaluation of achievement in environmental management of the Eco-School program, 6) environmental commitment, 7) environmental training, and 8) the program's materials and resources. The analysis of the results was carried out globally according to the dimensions assessed in the survey and by determining statistical differences significant for the two samples of eco-schools. The assessment of the results revealed substantial differences between the Chilean and Spanish programs' current level of activity, structure (leadership, teacher commitment, environmental management, training and production of materials) and the conditions for their maintenance, development and improvement.