Why do parties offer environmental policies in their political programs? While a number of papers examine the determinants of citizens' pro-environmental behaviour, we know little about the extent to which political parties adjust their platform towards environmentalism. We investigate this process through data provided by the Manifesto Project Dataset (CMP) for 20 European countries over the period 1970-2008. Following the literature on public concern towards environment, we examine economic, environmental and political determinants. Our findings provide evidence that political parties' environmental concern is strongly correlated with their political ideology and with country-level economic conditions.