For some time now, Rotterdam has actively portrayed itself as a policy laboratory. This laboratorial rhetoric, as one could call it, has prevailed in the fields of housing, urban problems and welfare provision, and most recently it has also emerged in the context of Rotterdam as a ‘smart city’. The latter is nothing special, as technological applications to urban problems are full of ‘urban labs’, of experimentation and of what Halpern et al. (2013) have called ‘test-bed urbanism’. However, in the context of urban and social policies in a very general sense, it is less common today. To understand the development of governing diversity in Rotterdam, it is pertinent to scrutinize the character and historical roots of Rotterdam’s laboratorial logic.