This chapter focuses on voter loyalty, looking at the barriers to entry that challenger parties face, most notably the strength of party attachments to dominant parties. First, it explores the development of party membership, which is the most formal expression of an attachment to a party. Second, it considers subjective measures of party attachment, which show cross-national volatility, but also clear signs of loosening ties. Third, the chapter studies actual behavior—namely, voter volatility, which captures individual-level party switching between elections. Here one sees that voters in the countries examined have become much more willing to switch parties. Finally, the chapter highlights the importance of different barriers to entry by comparing British and Danish case studies. Overall, the evidence suggests that voters are becoming more like consumers and more willing to switch if there is something more appealing on offer.