Between the late nineteenth century and early twentieth century, couples all around Europe began to deliberately limit their offspring, leading to an unparalleled decline in fertility levels. Even though these transformations affected all European societies, they did not happen for everyone in all places at the same time. Changes in family behaviour typically started in certain groups in specific places and later spread to the rest of the population. The underlying mechanisms of the spread of new fertility trends, however, are still a matter of debate. On the one hand, it could be argued that the determinants of fertility change – such as economic and cultural shifts – do not happen simultaneously everywhere. On the other hand, it has been suggested that there could be something more behind the patterns of change: diffusion processes, via the exchange of ideas and information between people, could affect the spread of new behaviours. The main purpose of this thesis is to investigate the underlying mechanisms of the spread of modern fertility behaviour in Belgium during the fertility transition. To this end, the research relies on a unique body of data gathered from different sources between 1886 and 1934. The datasets offer a high level of spatial and temporal precision and combine macro and micro perspectives. Different quantitative methods are employed in order to investigate the impact of diffusion on the timing, the pace and the geography of fertility decline. ; (POLS - Sciences politiques et sociales) -- UCL, 2015