The lean management philosophy is applied to improve the productivity and customer satisfaction of an organization. Besides themanufacturing sector,where it has been a standard for a long time, it was adopted by other sectors like service, administration, and research. Though lean implementations often fail or are not sustainable. Failure reasons and success factors are named based on literature reviews. Furthermore, businessmodels and awarding prizes for lean approaches are described and analyzed. Based on this, an assessment tool is developed. It consists of 24 criteria that are divided in the 4 categories: leadership, culture, knowledge, and process. The category culture is subdivided in: improvement, empowering, and partnering. The rating of each criterion works by interviews on the management and supervisor level, and surveys on all levels of the company. To get an objective third-party feedback, external experts can be consulted. The highest possible score is 10 and the lowest possible 1. To improve the score of low rated criteria, different countermeasures for each criterion are named. By gathering the knowledge of different research and case studies, this tool can help companies to implement lean in a sustainable way. Still it has to be considered, that every lean approach is individual and the proposed countermeasures have to be used properly. This tool is not verified in practice yet and needs of further research are mentioned as well.