Transnational Catholic nongovernmental organizations (NGOs) are among the most active agents in the promotion of the global common good as they seek to overcome the structures of sin that divide the human family. This dissertation investigates the theological and ethical significance of Catholic NGOs by developing a critical framework that uncovers the relationship between these organizations and the church's mission. Part One considers the global context and theoretical foundations of Catholic NGO action by examining social scientific literature (Chapter One) and modern Catholic teaching on the relationship between mission and justice (Chapter Two). Part Two places the theoretical foundations into dialogue with two case studies--the International Movement of Catholic Students-Pax Romana (Chapter Three) and the Jesuit Refugee Service (Chapter Four). This critical investigation of both theory and praxis illuminates several missiological, pneumatological, and ethical conclusions that are addressed in the final part (Chapter Five). This dissertation asserts three conclusions regarding the theological signifigance of Catholic NGOs. First, in contrast to some interpretations of the role of the church in the world, the actions of Catholic NGOs for the global common good are an integral part of the church's mission. Second, these organizations can be described as structures of grace as they embody charity and charism in their efforts to overcome the divisive effects of structural sin. Finally, a more robust awareness of the theological dimensions of their work can aid these and other organizations respond more effectively and ethically to the demands of the global common good today.