Despite the low success rate indicated by scholarly assessments, economic sanctions remain a commonly used foreign policy tool. Why do policymakers often turn to economic sanctions with great hope and enterprise in spite of their unimpressive success record? What determines a sanction outcome? Does the internal political dynamic of target matter in this case? How does it relate to different regime type? Hence this thesis examines the conditional relationship between the presence of domestic political opposition in the target state and sanction success conditional on the regime type by using data covering 763 US-imposed sanctions from 1945 to 2006. The findings suggest authoritarian regimes are more vulnerable to sanctions than their democratic counterparts in the presence of internal dissent groups in most cases. General Strikes are the best strategy to aid sanctions and coerce policy changes in authoritarian regimes. The presence of Guerrilla Warfare has the greatest substantive and statistical impact on sanction success. Consistent and organized internal dissent groups pose treats to the authority by weakening domestic stability or partnering with sender countries to push for policy changes.