The aim of this thesis research is to find ways to support student entrepreneurs in the development of sustainable startups. Specifically, to help them to oversee the sustainability impacts during the ideating phase. A design science approach is adopted for this. Seven experts on sustainable entrepreneurship related topics and eight student entrepreneurs working on sustainable startups are interviewed. In addition, twenty entrepreneurs provide information on their experiences in a survey. Ten design requirements are deduced from the content analysis of these research activities. The design requirements are then used to compare the tools/methods mentioned and to design a new tool. This tool is evaluated in a focus group with six student entrepreneurs working on sustainable startups. It is concluded that a major challenge for the student entrepreneurs is to determine in an early phase whether the value proposition of their startup idea makes positive sustainability impacts. This problem can be broken down into smaller ones, such as there not being a clear definition of what a sustainable startup is, there (consequently) not being a proper way to assess the sustainability, and the lack of well-supporting tools and methods in the ideating phase. The results of this study suggest that sharpening the definition of a sustainable startup and taking a systems perspective on the impacts helps the student entrepreneurs. To address this, a set of three Sustainable Startup System Mapping (SSSM) worksheets are proposed. This tool can be seen as a first step towards taking a systems perspective in the development of sustainable startups.