This chapter analyzes the effects of candidate selection methods, electoral systems and the interplay between them on party unity. It suggests that one must take into account the effects of both intra-party and inter-party competition, and the interaction between them, in order to better explain legislators' attitudes and behavior. Using attitudinal data from the PARTIREP project combined with data collected on candidate selection methods in 34 European parties in 10 countries, the chapter shows that the candidate selection method – the first of two barriers legislators must pass in order to be reelected – conditions the influence of the electoral system on legislative behavior. It examines several hypotheses concerning the effect of candidate selection methods and electoral systems, and the interaction between them, on legislative behavior. The pooled logistic regression models include four control variables: party agreement, parliamentary seniority, government participation and gender.