In this chapter introduces a new dataset capturing positions of German parties from 1990 to 2013 across sixteen policy domains and different stages of the representation process. It distinguishes between positions in the electoral arena and in each legislative year of the parliamentary arena. The chapter describes the dataset’s structure and highlights features that are key for further analysis. Using this new data, the chapter empirically tests the unitary actor assumption. The results reveal that differences among MPs of the same party are much smaller than differences between parties, thus supporting the unitary actor assumption. Finally, the chapter addresses the book’s first main research question: Do parties change their positions between the electoral arena and the parliamentary arena? Comparing party positions reveals that they change more strongly between arenas than within them. These findings highlight that party positions are not stable, and that parties deviate from their electoral positions when in parliament. The subsequent chapters analyse why this is the case and the consequences for voter representation.