The publication is based on a PhD thesis submitted at the Universities of Luxembourg and Passau (2012). It aims at presenting a historical geography of the city of Phnom Penh, comparing urbanisation processes constitutive of the colonial city in the past with those shaping Cambodia’s metropolis after the country’s independence in 1953, examining analogies and structural similarities as well as differences. The particular focus of this diachronic analysis is on the tension between planning and spontaneous ordering. Based on extensive archival material, which has never before been subjected to scientific scrutiny nor even – in some cases – been accessible to scholars, dating mainly back to the time of the French Protectorate, two distinct phases of development and marked caesurae in the urbanisation process have been identified. The first phase (1860s to 1953) is characterised by the establishment of Phnom Penh as colonial city, functioning both as headquarters of the French administration of the Protectorate and as main seat of the Khmer kingdom, gradually waning in influence. The second phase concerns the “rebooting” of Phnom Penh: the “resettlement” of the city after the forceful eviction of the urban population by the Khmer Rouge in 1979. This radical de-urbanisation during the Pol-Pot-era and the subsequent re-urbanisation constitute an exceptional case in the recent history of Southeast Asia, which – from an analytical perspective – offers a unique opportunity to study the following key issues: Which concurrent planning regimes operated during the colonial time, entailing what kind of “social engineering” and which frictions? Which spontaneous orders can be observed at the same time, but mainly during and following the resettlement of the city? The in-between phase, from Cambodia’s independence in 1953 to the coming into power of the Khmer Rouge in 1975, will also be examined regarding aspects of city development and planning under the king and later ruling “prince regent” Sihanouk, the short-lived “République Khmer“ as well as the “geometrisation of the country”, de-urbanisation and ruralisation under Pol Pot’s (un)real socialism.