In the early 1990s, the environmental work in the private sector around the world started to become less driven by legislative requirements and compliance and more by internal strategy. A development begins where environmental and sustainability issues have greater and greater strategic importance for business, and different approach in terms of concepts, methods or tools are starting to be created with the purpose to systematically improve business performance. The tool that has undoubtedly become the most popular in the1990s and the 2000s with companies to effectively and systematically assure compliance, develop environmentally more efficient processes, and gain increased competitiveness, is environmental management systems (EMS). Many of the implemented EMSs are based on the international standard ISO 14001. As EMSs has increased in popularity the interest in the world to evaluate the systems financial, environmental and organizational effects has grown. Several evaluation studies have been conducted both in Sweden and internationally. Some of these studies argue that EMSs lead to improvements while others are more hesitant in their conclusions. With few exceptions, the studies have relied on company representatives perceived benefits of EMSs. Subjectivity is problematic since research shows that business representatives often have reason to give a positive image of their company. Since the studies based on subjective assessments tend to show more positive effects of EMSs than the few studies based on real environmental data, new studies should be based on objective data in the form of various types of environmental parameters. In addition, evaluation studies rarely focus on the systems impact on regulatory compliance, so there is every reason to continue to study this. The impact of the EMSs on compliance is an interesting issue for environmental monitoring authorities. Sweden has chosen not to impose regulatory relief for companies that have introduced EMSs while this is done in several other countries in Europe. To help with decisions regarding the situation in Sweden, the purpose of this study to study if Swedish ISO 14001-certified companies improve their compliance at a higher rate than other companies. This study focuses on change in environmental performance over time as EMS is a tool for continuous improvement and does not say anything about the level of performance. Furthermore, the study includes not only companies with EMSs but also business without systems for comparison. Finally, the improvements in the companies before the introduction of EMSs are taken into account through the use the historical rate of improvement in the analysis step of the study. The analysis in the study is based on credible environmental data from the environmental reports that companies with environmental permits must submit to the regulatory authority every year. Data has been collected from 66 companies that were ISO 14001-certified in year 2000 and 50 randomly selected non-certified companies. Comparisons have been carried out between the two groups of companies using established statistical methods, especially t-test. Comparisons have been made over the periods 1997-2003 (short term) and 1994-2006 (long term). Comparative analysis has been carried out for nine different areas: air emissions, water emissions, resource consumption, energy consumption, waste, parameters in the companies' permits, parameters not included in the permits, the overall environmental performance and violations and incidents.The analysis of various environmental parameters in the nine areas shows that overall there is no statistically significant difference of changes in the improvement rate between certified and non certified companies at 95% confidence level. The results show that in most cases we have a situation where the degree of suspicion that there is a difference between groups is low. However, we have on several occasions a situation where we may suspect that a difference between the two groups exists. In some cases, the results give us reason to suspect that the certified companies are increasing their improvement rate faster than those not certified (e.g. energy consumption and waste). In some other cases, the situation is reversed (e.g. air emissions and overall environmental performance), which means that the image of environmental management systems effects are not uniform. Consequently, it is difficult to give any recommendations for operational monitoring authorities how to take into account the existence of certified EMSs in the monitored objects based on the direct results of the study. Possibly, one could propose that, in the certified companies, one could focus monitoring activities on emissions to air and water while one do not need to focus as much on the energy and waste areas. Unfortunately, the results regarding compliance and overall environmental performance are divergent and sometimes also difficult to explain and it is therefore difficult to come up with some concrete recommendations. However, the results of this study together with results from other studies give us a picture of the impact of EMSs on corporate environmental performance. In some cases, the systems have positive effects on environmental performance but in most cases it is not possible to find any effects at all. In the cases we see positive effects on environmental performance the improvements are relatively small and limited. In other words, companies planning to introduce an EMS and move towards certification should not expect any dramatically improvement in environmental performance.