The City of Copenhagen aims to become the cleanest capital in the European Union, and one of the cleanest cities in the world. To achieve this goal a number of actions have been established. The goal is to educate and nudge residents, tourists and other groups of people living, working or visiting Copenhagen, to use waste bins and refrain from throwing their garbage on the streets. (Status på København 2016, 2017) The City of Copenhagen also wants to become a Smart City and have begun looking into various solutions using people generated data. Tracking the flow of traffic is well known, but the city has begun looking into solutions using tracking technology, too, so e.g. waste bins themselves notify the city’s cleaning crews when it’s time to empty them. (Street Lab, 2017). A smart city should be a clean place for the residents and visitors to spent time in. In the City of Copenhagen 20% of the residents are children and young people under the age of 18. They live, learn and play in the city. The City has taken a special learning initiative targeting school children: A learning program; The Mobile School of Litter and Sustainability in Copenhagen (MLS) (Uddannelse for Bæredygtig Udvikling, 2017). In this program children age 6 to 16 learn about garbage and resources in school. A session with the learning program starts in the classroom with general talks about garbage and litter. Then the children are sent out in larger groups of 6-8 children into the local area around the school to collect litter, so they can experience the challenges of litter first hand. Here they are accompanied by educators from MLS and their teachers. The tools they bring are grabbers, and a garbage bin on wheels. Certain types of litter they are not allowed to pick up, e.g. syringe needles, broken glass and hazardous biological material. Back in the classroom and with the help of MLS educators they talk in general about garbage and litter and reflect on the environmental and economic aspects of littering. The children generate many different data during the session and the older children, 3rd grade to 10th grade carefully write down their findings on paper, and analyze the data in the classroom. However, at present these data do not go any further than the children and the classroom. Our goal is to make the pupils (user generated) data on litter findings useful to the pupils after the session ends, but also useful beyond the classroom and for the City. We do this by designing an app: Skolen ta’r Skraldet (Litter in English), to be downloaded and used by the children on their own mobile devices. The app will register the children’s user generated, location based data, contributing to their learning, and at the same time, the registered data is stored and accessible to the City, hence the data can be a part of the Copenhagen Clean and Smart City Solutions. (Copenhagen Solutions Lab, 2017) With basis in the social constructivist thinking we frame our work through Dewey´s theory of Learning by Doing (Dewey, 2005) together with Lave and Wenger’s theory on Situated Learning (Lave & Wenger, 2014) to inform our understanding of children’s´ learning by doing, in collaborative teams. We use a Design-Based Research approach (Dalsgaard, Pedersen & Aaen, 2013) in connection to HCIlifecycle when designing the prototype Skolen ta’r Skraldet. In this master’s thesis we introduce the Learning Program, the target group of children and document our design of the prototype. We report on an explorative study of the introduction and use of the app by 2 a group of young children, their experiences with the app and with collecting litter. In connection with the different chapters we reflect critically on our design, the learning potential in connection with Copenhagen Smart City thinking and the results from our explorative study. Keywords: location based learning, context based learning, Smart City, litter, sustainability, Design- Based Research, and pupils’ user generated data, Human-Computer Interaction.