For some time now, students of the US Congressional parties have debated whether party cohesion is largely a function of the leaders successful efforts to impose discipline on their followers or merely a product of the shared preferences of party members. Recent comparative contributions to this debate show that parliamentary institutions can and do supply party leaders with the capacity to enforce discipline even when their members disagree, and that discipline is a necessary aspect of party cohesion in any multiparty setting. However, party leaders exercise their influence over a group of people who have:first, chosen to join the party;second, whom the party has agreed to accept; and third, on whose support the leader may rely. Political scientists should focus their efforts on understanding the connections between party cohesion and discipline on one hand, and the politics of candidate and leadership selection on the other.