This paper performs an analysis of tourism seasonality in Spain, typically from a market-side perspective. Three exercises are performed; firstly, seasonality is analysed through monthly concentration indexes and, in particular, the CV; secondly, the role of markets is explored based on an additive decomposition technique; thirdly, its main economic determinants are assessed through a dynamic panel data model. The main results obtained can be summed up in: first, seasonality in Spain has clearly worsened since 2008, coinciding with a strong growth in overall demand; second, three markets generate two thirds of global seasonality, with the pattern of the UK market of particular concern; third, aggregate demand models suggest that prices, exchange rates and especially income levels are significant explanatory factors. Keywords: seasonality; markets; dynamic panel data models; Spanish tourism