The study takes its point of departure in the urgent problem of slums that follow on the rapid urbanisation worldwide. Focusing on the small informal settlement of Kambi Moto in Nairobi, Kenya, the study tries to answer the question of how power can be worked out in slum upgrading – a way to change the physical environment of a slum without demolishing and rebuilding the whole settlement. The theoretical tool to answer this question is taken from Edward Soja’s reading of Henry Lefebvre in the concept Thirdspace – an extended and politicised way to look at space, where space is not only seen as a stage for historical and social processes, but as something that is shaping our thoughts and actions; a social space that includes and goes beyond the material Firstspace and the mental Secondspace. From a spatialized reading of history today’s situation – where 60 % of the population of Nairobi live in informal settlements – is traced back to the ideological structuring of space in the colonial cityplans. The informal settlements are established as a Thirdspace: both a negative outcome of the dominating Secondspace of the colonial administration and as a counterspace, where traditional ways of life could live on and where revolutionary movements could grow. The study then focus on how the two scales to view the city, the macro and the micro, are resolved in the Shack/Slum Dwellers International (SDI), a global network of local federations that organizes slum dwellers. The network empowers the individual slum dweller in making him/her an actor in a peer to peer exchange, and also creates a social space for political struggle. This is manifested in Muungano wa Wanavijiji, a citywide movement for a collective struggle for spatial rights, empowering the slum dwellers in taking charge of the social production of human spatiality. In a case study of a slum upgrading effort in Kambi Moto the shifting of power from the government, international organisations and professionals to the lived Thirdspace of the habitants, as well as the internal power relations within the community, are looked at in a concrete situation.