Today, knowledge is addressed as a key driver in urban development. From an urban planner’s perspective, however, it seems that the knowledge we talk about is out there in so-called knowledge industries. Knowledge-based urban development refers to development of city regions that are more or less driven by the knowledge economy, or to opportunities to attract knowledge workers in order to fuel economic growth in specific areas. The aim of this chapter is conceptualizing what knowledge and learning mean to, and in, contemporary planning praxis. The chapter discusses the key concepts of knowing and their relation to doing. By mooting a theory of assemblages, the chapter further provides a foundation for the analysis and the enforcement of learning in urban development praxis. Drawing on research on learning organizations, this chapter provides a basis for the contribution of urban planning to knowledge-based urban development.