Tourism seasonality is generally seen as a problem for most of the main destinations in the world, particularly from the point of view of sustainability. Despite its importance, no reasonably homogeneous international measurement of seasonality is yet available on the global scale. Using the best UNWTO data, the paper uses the Coefficient of Variation (CV), over the period 2008–2013 and for the main destinations, as a measure of tourism seasonality. In addition to the descriptive results, the paper includes a mixed effects panel data model, which allows us to investigate some reasonable main global determinants of seasonality. The key results obtained may be summarized as follows. Firstly, the world seasonality shows an inverted U pattern, trending upwards until 2011 and subsequently descending. Secondly, in contrast to other regions, the highest (and increasing) seasonality is concentrated in the Mediterranean countries. Lastly, in terms of empirical determinants, geographical location, and the income of the major markets of origin are globally significant variables. We believe that these results, beyond their academic value, may be useful for policy-makers. Keywords: Seasonality; concentration; measurement; determinants; panel data; worldwide