Quantitative data collected during a recent research trip have shed light on the social and linguistic factors that affect language choice and use in the complex multilingual setting of Nairobi City. The first section of this paper provides the sociolinguistic backdrop of Nairobi, addressing issues of language choice among the city residents, the general distribution of Kenyan national languages in areas of the city, and functional uses of those languages as well as Swahili and English. The main section of the paper however focuses on the widely used mixed code known as 'Sheng'. Used principally among the youth, it is based on Swahili grammar but uses resources from other Kenyan languages to create a dynamic, mixed code. Stratified data samples were collected during a period of four weeks in selected city locations covering the four compass directions. While the larger part of the survey consisted of quantitative information in the form of a written questionnaire, raw speech data collected during open-ended interviews are used to sketch a structural description of Sheng in areas of morpho-syntax and phonology. These are then contrasted to Kenyan Standard Swahili. Attitudes towards Sheng are also considered, and compared to Swahili, English and other langauges spoken in the city. Present and future implications of the use and spread of Sheng in urban Kenya raise a few questions about the impact of Sheng in primary and secondary school, its implications for the growth of Standard Swahili, and (lack of) language policy in Kenya.