This paper analyses the seasonal concentration of tourist activity in the main Spanish provinces for the period 1999-2012, taking hotel nights as the indicator of reference. We propose using several standard summary measures in order to evaluate the level, evolution and some decompositions. Our main results can be summarized as follows: first, across the whole country and especially since 2007, there is a growth in seasonality; second, seasonal concentration is greatest in the Balearic Islands and two of the Catalan provinces, and least in Madrid and the Canary Island provinces; third, although the overall patterns typically agree, nevertheless, in some provinces the indexes we deal with show some discrepancies; fourth, the decomposition of the monthly concentration by major markets typically indicates the main role played by the foreign component; finally, the overall evidence does not support the thesis that the domestic market offsets the foreign one. Keywords: seasonality; concentration indices; decomposition analysis; Spanish provinces