Around 5000 agricultural biogas plants were operating in Germany by the end of 2009. There is a recognized need for their assessment, comparison and performance improvement. Prerequisites for their reliable assessment include: 1) detailed and reliable performance data, 2) defined criteria for the assessment, and 3) comprehensive method. The limitations of existing assessment methods have been reviewed. The new method developed for assessing the overall performance of biogas plants is focused on four assessment aspects: biogas production, biogas utilization, environmental impact and socioeconomic efficiency. Each assessment aspect was determined by two performance criteria. The method is based on fuzzy set theory and fuzzy mathematics, which enables use of imprecise and uncertain data. Incorporation of enough expertise from the field of biogas technology was enabled in this method as well. Pre-assessment of future biogas plants in their planning phase is possible, contributing to their effective development. The reliability of the method was tested using data from ten monitored biogas plants. The assessment of the ten biogas plants showed that biogas utilization is the aspect with the largest potential for performance improvement, by increasing the external heat utilization. The method developed is adjustable to accommodate new developments related to biogas technology and biogas plants in any geographical region. Future research should be focused on definition and inclusion of more assessment criteria and on developing the methods that are capable to handle compensation between individual criteria. (C) 2011 Elsevier Ltd. All rights reserved.