The subject matter of this research is to examine the following questions: Already efficient companies adopt Corporate Social Responsibility (CSR) practices as a marketing tool and as a means to promote themselves? And at the same time, already efficient companies incorporate CSR in their strategy, thus reinforcing their sustainable development? Specifically, our research is comparative and involves listed companies in the Athens Stock Exchange which have adopted CSR and those who have not adopted it, for the period 2006-2012. In particular, it compares these two company types: listed companies which implement CSR practices and listed companies which do not implement CSR practices, with respect to efficiency ratios, which as explained also in the methodology, are ROE, ROA, EPS, ROS1 & ROS2. A key conclusion of the research is that companies which adopt CSR have higher average efficiency compared to those which do not adopt CSR, and also, already efficient companies use CSR as a means to promote themselves while incorporating CSR practices that drive their Sustainable Development. Something illustrated by the fact that the higher percentage of already efficient companies which implement sustainable development policies, is involved in all four CSR action fields: workplace, market, community, and environment.