This chapter outlines the main approaches to the effects of political communication within media studies, and examines the evidential bases of effects research. It considers the extent to which the purposeful communicative behaviour of political actors, such as political advertising and conference speeches, can influence the attitudes and behaviour of the intended audience. Effects of this type can be examined at the micro-level of the individual consumer of the message, or at the macro-level, when individual responses to political communication are aggregated together in the form of public opinion polls and other indices of collective political will including elections. The chapter examines how the political process of democratic societies - their procedures and practices - has been affected by the growing importance within them of mass media. It also considers the systemic impact of the rise of political communication on societies around the world.