The Pentecostal Movement, currently the second largest Christian constituency after the Roman Catholic Church, is the most rapidly growing Christian movement in history. It has grown from zero to almost 500 million in one century. Pentecostals are known for energetic evangelism and mission. They are often accused of proselytism and the lack of social concern in their mission. The International Dialogue between Pentecostals and the Roman Catholic Church has dared to have another look at these vital issues affecting the rest of the Church. The fourth round (1990-1997) of the dialogue, started in 1972, focused on mission, proselytism and common witness. The present study, written at the Institute for Ecumenical and Cultural Research (Collegeville, Minnesota, USA), under the mentorship of Kilian McDonnell, OSB, the co-founder and co-chair of the International Roman Catholic-Pentecostal dialogue, highlights the similarities and differences in mission and related issues between these two largest Christian families.