Children and youth’s involvement in participatory urban planning processes is a phenomenon that is given increased attention by planning actors and stakeholders, as well as it is gaining space in research regarding environmental- and social sustainability. This thesis research examines how children and youth’s participation is defined, and implemented, in the two major Swedish cities of Stockholm and Gothenburg. The main challenges, and current possibilities, for planning professionals in the two cities are explored. Drawing on qualitative interviews with experienced planning professionals, and relevant documents, I analyze current issues with defining and implementing children and youth’s participation in the practice of urban planning. Discerning participation from consultation, or simply asking about children and youth’s opinions, is surrounded by some confusion. This occurs in the phase of defining and implementing children and youth’s participation, in both theory and practice. There are also factors in the formal institutional context which pose challenges to implementation, since time frames, budgets, and resources to develop knowledge and competence are limited. Other challenges planners face relates to ways in which they can use the information collected when engaging children and youth, and how to provide feedback to participants as a part of daily practice. Participatory planning projects allow for learning to take place, inclusive of planners, children and youth. Children and youth contribute with new perspectives and insight about their surroundings, which are important to include in urban planning. Participatory processes enable them to develop skills of becoming community builders and democratic citizens.