Many non-profit organizations (NPO’s) in the Netherlands have lost a large amount of donors over the last years. This thesis explores the use of behavioural influence design, applied in an online solution, in order to improve donor engagement for the well-known NPO WWF. The thesis focuses at creating long-term engagement with donors in the age class between 18 and 35. People in this target group want to be flexible when it comes to making donations, want their own impact to be specific and tangible and rather take action themselves, in a personal and individual manner, when it comes to environmental goals. Furthermore, this is an age group that is very active online, especially on social media, which is interesting for the online solution space. As WWF-NL wants to create a society-wide movement, showing people that there are more ways to contribute to their mission for a sustainable society, beyond donating (like adjusting sustainable behaviour or investing time), it seemed an interesting opportunity to improve engagement with this group and connect with them on the common interest of contributing beyond donations. With the help of literature and successful exemplar cases on engagement, guidelines haven been created for improving the different levels of engagement (cognitive, emotional and behavioural) in relation to the target group. Also, the field of behavioural economics, which assumes the irrational thinking and decision making processes of human beings, has been explored to see how behavioural influence design could be applied to improve the (different levels of) engagement. The conclusion of this exploration is that behavioural influence design should not (just) be implemented in the smaller details of online expressions; it should be part of the broader strategy, where it is used to improve individual (in this case sustainable) behaviour of the target group. Again, with exemplar cases and literature, guidelines for improving engagement with the help of behavioural influence have been created. The guidelines and insights for improving engagement with the help of design for behavioural influence are combined in a strategy. Its details, that make it a WWF-specific approach, are shown in a roadmap, with supporting directional concepts. To successfully reach the vision of this roadmap, all parts of the horizon must be followed carefully. The remainder of the project focused on horizon 2 of the roadmap, to provide WWF with a specific and tangible project to pick up and carry on after my graduation; the development of the WWFxYOU app. The goal of this app was to create a personalized relationship between the target group and WWF, based on the motivation to create impact (on an individual level). A first test, with 10 in-depth interviews with participants from the target group, had promising results; the app was described as a personal, approachable, (inter)active and involving way of contributing to WWF and seemed more attractive than current (WWF) donorships. A redesign is made based on the interviews. For further validation, I recommended to improve and test this design with a larger participant sample, before actual implementation.