Asthma is becoming increasingly frequent in children in urban and rural areas of low and middle-income countries (LMICs), with a high prevalence in urban centres. The urbanisation process has been suggested as a possible explanation for this temporal increase and urban-rural differences in asthma prevalence. This thesis aims to explore the influence of urbanisation on asthma prevalence in a developing region of Latin-America in which the rural-urban transition is occurring rapidly. A systematic review evaluated how epidemiological studies have assessed the associations between asthma and urbanisation in LMICs. To understand better the relationship between urbanisation and wheeze/asthma in transitional areas, four cross-sectional studies were conducted in northwest Ecuador. In the first of these, the level of urbanicity at census ward level was quantified for children living in diverse urban and rural localities and urbanicity level was associated with asthma. The second and third study explored how internal migration, an important element of the urbanisation process, is associated with the prevalence of current wheeze in urban and rural populations, respectively. The fourth analysis identified lifestyle domains in urban and rural populations based on socioeconomic and behavioural characteristics and explored how these characteristics could explain the prevalence of wheeze in a transitional region. This thesis provide evidence that even small-scale increases in levels of urbanicity are associated with a higher prevalence of wheeze/asthma. Analysis of internal migration showed that in urban areas, rural to urban migration was associated with an increase in the prevalence of wheeze while in rural areas the absence of the child’s mother at home, through temporary or permanent migration, was associated with an increase in prevalence of wheeze, rhinitis and eczema. Finally, an analysis of lifestyle factors showed that living in substandard housing and a high level of sedentarism were associated with a greater prevalence of wheeze.