Simon Kretz and David Chipperfield (Eds.): On Planning – A Thought Experiment. Verlag Walther König 2018. This publication is the result of a year-long dialogue between David Chipperfield and Simon Kretz, enabled by the Rolex Mentor and Protégé Arts Initiative. It critically thematises and theorises urban qualities of contemporary urban developments. Using the Bishopsgate goods yard site in East London as a case study, and with collaborative input from the Institute for Urban Design, Prof. Kees Christiaanse, ETH Zurich, this project highlights the conditions under which an ideal urban dev elopment project could fl ourish. The conclusions reached through this exercise demonstrate how future large-scale developments elsewhere could have more positive urban impact, both at the scale of the neighbourhood and the wider metropolis. It debates the contemporary planning culture and provides a vehicle to discuss the relations between the development of a particular site and the city in its entirety, between urban processes and resulting forms, between active and passive planning, and their political and ethical dimensions.