Abstract: Public policy is the result of strategic interactions between political actors. In this dissertation, we study different manifestations of such political influence. In the first chapter, we focus on influence within political organizations, analyzing the determinants of the balance of power between a party leader and party backbenchers (i.e., party discipline). The model formalizes the tradeoff between resources at the leader's discretion, and her need to maintain a minimum level of support to continue leading. We show that offers of publicly observable, irreversible payments on the spot increase the value of promises of future partisan benefits such as nomination to party lists. Moreover, contrary to conventional wisdom, these promises are insufficient to grant significant power to the party leader. In the second chapter, we focus on influence between branches of government. In particular, we examine empirically the political incentives faced by individual justices of the Argentinean Supreme Court. While Argentina's constitution and electoral rules promote a fragmented polity, most analysts do not consider the Argentina judiciary as independent. We show that this perception is inappropriate. Our results show an often defiant court subject to constraints, behaving strategically. The probability of voting against the government falls the stronger the control of the president over the legislature, but increases the less aligned the justice is with the President. In the third chapter, we focus on the influence of interest groups on public policy. We link the theory of interest groups influence over the legislature with that of congressional control over the judiciary, and study the implications of separation of powers for the existence and effectiveness of lobbying by interest groups. The resulting framework reconciles the theoretical literature of lobbying with the negative available evidence on the impact of lobbying over legislative outcomes, and sheds light to the determinants of lobbying in separation-of-powers systems. We provide conditions for judicial decisions to be sensitive to legislative lobbying, and find that lobbying falls the more divided the legislature is on the relevant issues. We apply this framework to analyze Supreme Court labor decisions in Argentina, and find results consistent with the predictions of the theory.