We develop a formal model of opinion polls in elections and study how they influence the voting behaviour of the participating agents, and thereby election outcomes. This approach is particularly relevant to the study of collective decision making by means of voting in multiagent systems, where it is reasonable to assume that we can precisely model the amount of information available to agents and where agents can be expected to follow relatively simple rules when adjusting their behaviour in response to polls. We analyse two settings, one where a single agent strategises in view of a single poll, and one where multiple agents repeatedly update their voting intentions in view of a sequence of polls. In the single-poll setting we vary the amount of information a poll provides and examine, for different voting rules, when an agent starts and stops having an incentive to manipulate the election. In the repeated-poll setting, using both analytical and experimental methods, we study how the properties of different voting rules are affected under different sets of assumptions on how agents will respond to poll information. Together, our results clarify under which circumstances sharing information via opinion polls can improve the quality of election outcomes and under which circumstances it may have negative effects, due to the increased opportunities for manipulation it provides.