Ensuring environmental sustainability is the aim of the seventh Millennium Development Goal (MDG7). United Nations data shows insufficient progress, especially in developing countries. In order to generate new information about this progress and the main barriers that hinder it, we conducted a systematic review to describe the scientific evidence concerning MDG7, and to analyse the quality of the studies. In order to identify all empirical quantitative studies (ecological, descriptive study, cross-sectional, case-control, and cohort), we searched the following databases between 23 and 26 June 2009: PubMed, EMBASE, ISI WoK, SCOPUS, EBSCO-Greenfile, CAB Abstracts, and LILACS. We identified six empirical studies which reported serious difficulties in the progress of MDG7 and identified socioeconomic inequalities, lack of infrastructure and contamination of water sources as the main barriers hindering its achievement. Environmental issues do not seem to be a priority in research and political agendas, while the international organisations continue to insist that the objective is extremely important. It is necessary to generate scientific knowledge around MDG7, and identify weakness in the indicators in order to adapt policies to the new challenges and make decisions based in best evidence.