A trend in current society is reducing the environmental impact of products. One of the industries in which this trend is noticeable is the yachting industry. Yachts with a lower environmental impact are being demanded. Furthermore, regulations are being put in place that are likely to limit the effect yachts can have on the environment. One problem arising with this demand is that there is no clear indication of what the environmental impact of a yacht is. Therefore, the concept of the Yacht Environmental Transparency Index (YETI) has been developed. To ensure the YETI will be used in the industry, it is proposed that the development of this index be done by yacht builders and designers. However, there is no structured process how this group of stakeholders can develop a credible environmental impact index. Therefore, the goal of this research is to develop a process design towards a credible environmental impact index for yachts. The research is divided into five steps that contribute to the development of this process design and find an answer to the following main research question: How can a process design towards a credible environmental impact index for the operational lifetime of superyachts be developed? The research begins by defining an environmental impact index via a literature study. The literature study was undertaken to identify what must be developed by the yacht builders and designers and what must be the result of the process design. It was found that, to develop a credible index, a test procedure must form the basis of an environmental impact index. The second part of the literature study was executed to obtain insights into how a test procedure can be made credible. From the literature, seven principles have been identified that must be considered during the development of an environmental impact index. These seven principles can be divided into two groups. Where one groups contains principles that are regarding characteristics of the index that is developed, the other presents principles regarding the development process. The principles are not, however, specifically focused on the development of an index by stakeholders from an industry. Therefore, the IAD framework is used that structures the principles of credible index development and identifies the effects they have on the behaviour of and interactions between stakeholders during the development. Certain rules are provided by the framework that are coupled with principles. This process results in an overview of how a credible environmental impact index can be developed by stakeholders of an industry. The rules prescribe how the development process has to be governed and what has to be the outcome of the development process. The two groups of principles are assigned to the rules accordingly. From the IAD framework, it is apparent that the development of the index partly depends on the environment in which the stakeholders act and who is involved. Therefore, an analysis of the industry was executed to identify its influence on the development of an index. It was found that the technical characteristics of a yacht and the behaviour among yacht builders and designers complicate the development of an index. In addition to the influences of the industry, the development of the process design is based on the results obtained from a workshop with yacht builders and designers. This workshop had the goal to specify the rules assigned to the principles of environmental impact index development. These rules concerned how to develop a credible index that is capable of indicating the effect on the environment by a yacht during its operational lifetime. Based on these rules, a process design is developed. The process design consists of two aspects, the governance of the development process and steps that have to be executed in order to provide a credible index. The most important aspects of this process design are highly correlated with transparency. The credibility of the index is ensured when stakeholders develop it and reach a consensus on how the YETI should be developed. Any index that is developed must be made public, including the development method. A third party must verify whether what is included in the test procedure can be predicted and measured correctly and transparently. Also, standardisations must be developed for all procedures to calculate and verify the value of indicators included in the ‘test procedure’. Standardisations enable verification by stakeholders and therefore make the index credible. In general, credibility is objective and dependent on the perception stakeholders have of the index. The process design leads toward the YETI in such a way that that stakeholders are likely to perceive the index as credible.