In the last few decades, significant changes in livestock farming systems and land use were observed in European mountain areas with large implications for the sustainability of grazing agro-ecosystems. System dynamic studies become essential to understand these changes, identify the drivers involved and trying to anticipate what might happen in the future. The objectives of this study were as follows: (i) to analyse the main recent changes that occurred in mountain cattle farming in the Spanish Pyrenees; (ii) to typify diverse trajectories of evolution of these systems; and (iii) to establish drivers of change that might help understand the evolution of mountain agriculture. A constant sample of mountain cattle farms was analysed for the period 1990 to 2004. In total, 30% of farms have disappeared during this time interval. For the remaining farms, the most important general changes observed were as follows: increment of size; change of productive orientation from mixed beef-dairy to pure beef production; extensification of grazing management; reduction of family labour and increase of pluriactivity; reduction of unitary variable costs; and increase of labour productivity. After the elimination of common temporal effects between dates, multivariate techniques allowed for the identification of three patterns and six specific trajectories of evolution that are profiled in the text. Relationships between the patterns of evolution and other variables referring the farm, the household and the socio-economic environment were identified as drivers of change: (i) the specific location of the farm in relation to the capital village of the municipality and the evolution other sectors of the economy, in particular tourism; (ii) the size of the family labour, presence of successors and degree of dynamism of the farmer; and (iii) the initial orientation of production.