Previous research explains the evolution of parties' ideological positions in terms of decision rules that stress the uncertainty of the political environment. The authors extend this research by examining whether parties adjust their ideologies in response to two possible influences: shifts in public opinion, and past election results. Their empirical analyses, which are based on the Comparative Manifesto Project's codings of parties' post-war programmes in eight West European nations, suggest that parties respond to shifts in public opinion, but that these effects are only significant in situations where public opinion is clearly shifting away from the party's policy positions. By contrast, no evidence is found here that parties adjust their ideologies in response to past election results. These findings have important implications for parties' election strategies and for models of political representation.